Such a statement applies mostly subjectively, depending on the needs of every individual, the type of person you are, what you value, what you consider unimportant in your day to day life, and so on. This means that, depending on all these factors and more, there is knowledge that is important to you and that can help you grow and move forward in life, and also knowledge that could do nothing for you but to waste your time.
All this comes from a person who really likes to accumulate knowledge. I like to read books, I like to learn new things and to discover as much as possible. But despite all this, I also understand that trying too hard to understand and know everything that you come across, because you think that everything is important, is not only a waste of time, but also extremely exhausting.
I know this because years and years ago I was the type of person who tried to learn everything and know as much as possible. I would read dozens of articles every day and constantly find new things that I needed to learn more about and understand properly, from cognitive biases, to financial advice, psychology, history, biology, to more practical things, such as how to create a website, how to code an app, how to draw from different angles, how to repair a computer, restore one, and so on.
Everything I stumbled upon was important, and I always tried to make an effort to learn about it. But looking back on it now, I consider it all a huge waste of my time. Don't get me wrong - finding about those things was good. I now know that those things exist, and if I am ever in need of them, I know how to name them and search for more information. The waste of time was me trying to learn and, more importantly, remember everything.
I thought that the more I remembered, the smarter I would be. Now I understand that it's not knowing everything that makes you smart - but knowing the right things. Additionally, knowing that you cannot know everything and then learning how and where to find the right information whenever you need it about those topics that you're not that experienced with is way more important than trying to memorize every bit of information that you encounter.
Here's an example: recently I've been spending more and more time reading, both fiction and non-fiction. Right now I'm reading a book called "A short illustrated history of Romanians", in an attempt to understand a bit more about the history of my country. I know, for a fact, that I cannot remember everything that I'm reading. I also know that trying to will require a lot of time (which I don't have) and a lot of effort.
And yet, a lot of the information in the book is interesting. So what do I do with it? Do I try to remember it? Do I keep the book close all the time so that I can use it as a reference when I need to? Do I accept the limitations of my memory and I just go through it without blaming myself for not remembering everything?
No. What I do, first, is what I call "exploratory reading", which means that I read the book, beginning to end, without making an active effort to memorize anything from it. Whatever sticks, sticks and what doesn't, it's still in the book. Once I finish it and after enough time goes by, which in my case will probably mean once I get back home at the end of the year, after 7 months of working abroad, I will read the book again, but this time, I'll do it by paying extra attention to the bits that seemed interesting to me in my first read.
That second read does a few things. First, it reinforces the bits of information that I naturally found interesting in my first read. Then, it helps me remember additional information about those bits that I didn't care about that much at first. And lastly, and the most important part, I learn exactly where to find the information about what I can't remember naturally, in case I ever need it.
I don't need to remember all the dates and all the important people from the history of my country. What I need to remember, and what I will, is that the dates exist, that such people exist, and when I need to know more about either, I'll know in which book to look.
Exploratory reading is, for me, a way to discover more about the world and what's in it. I don't need to know every little detail about the history of the world - but if I can remember the important bits, and what happened around the time when they took place, I gain the ability to find all the information I need about the small things that I don't remember. The same thing applies to the history of my country.
Most importantly, the same thing applies to everything that we "explore". I don't need to know everything about cognitive biases, but knowing that they exist, and knowing what they are, gives me the ability to find more information about this topic if I need to. I don't need to know everything involving financial literacy, but knowing a few things about money, and how to invest, gives me the chance to learn more about those topics if I need to. Even knowing that the term "financial literacy" exists gives me the ability to read more about it if I want to. The exact same thing applies to every other topic.
This is very important to know because in this day and age everyone considers their knowledge to be the most important, and everyone thinks that what they just learned is going to change their lives. And so you're constantly bombarded with new information from everywhere, from people telling you that learning that particular thing will help you solve all your problems.
It's good to know that their information exists, and perhaps where to find more about it, should you need it. But aimlessly trying to remember everything, in the hopes that you'll be smarter than everyone is just a fruitless activity. It's better to focus on learning those specific things that are important to you, and then to just know what other things exist and where to find more about them, than to spend hours and hours every day trying to remember a bunch of things that you'll never use and that you'll forget within a few weeks or months.