There is a whole branch of philosophy dedicated to this question, it’s called epistemology. We use the word “know” in a number of different contexts. Here we are speaking of propositional knowledge. A proposition is a statement that something is, and it is a statement that is either true or false.
I prefer to use the word “claim” rather than “proposition.” It seems a little clearer and is more common. A claim, just like a proposition, is a statement that is either true or false. But knowledge is more than that. It is something that is believed. For something to qualify as knowledge it must be true and it must be believed.
But that is still not enough to qualify as knowledge. You can make a luck guess that the shirt I am wearing is white, but you did not “know” that. So guessing is not a part of knowledge. The classical definition of knowledge is “justified true belief.” The justification part simply means that you have good and reliable grounds to believe a claim is true. In other words, your belief is based on evidence.
But then a man named Edmund Gettier published an article in 1963 that called into question this classical definition of knowledge. These Gettier cases, as they came to be known, show that in some cases, someone might have a “justified true belief” that ends up being wrong.
For example (not used by Edmund Gettier), George gets up in the morning and looks at the clock in the Kitchen (the only clock he has) and it says 6:15. So he believes it’s 6:15. And it just so happens that it is 6:15. So he was a “justified true belief.” But the clock has stopped two days a go. It was simply luck that he looked at the clock at exactly 6:15 two days later. He does not have knowledge, argued Edmund Gettier. Almost all epistemologists have agreed.
The struggle to answer these Gettier cases continues. Some have tried to explain them away, others have tried to give a fourth qualification to knowledge. The solution is still not agreed upon. The answer may involve coherence of beliefs, or the truthfulness of assumptions. But each solution causes other problems and raises other issues.
I use the classical definition in my book Does God Exist? I would argue that knowledge is “justified true belief” because no better definition has yet been agreed upon. But I am aware that it is not always the case, as Gettier cases show.