Since Gettier, what is meant by knowledge is no longer justified, true belief. It is merely justified belief. The classic sense that we must know that something is true while also believing it is true, begs the question entirely. So, to know is to have a justified belief.
Defined, justification is a reason to believe one case rather than any other. This is called a “conclusive reason”. Synonymous is “reason to assert” and “warranted assertability”.
The obvious problem is that we can have many, equally good beliefs having equally good conclusive reasons to believe them. What’s not so obvious is that “being wrong” doesn’t equate to one believing something false. It entails that before we knew what was true, we were not justified to believe anything else, since had there been a reason to, we would have. As long as if believing what we were justified to believe was the most compelling thing to believe, we must believe it even though that belief may turn out to be mistaken.