Gettier, "Is Justified True Belief Knowledge?"
Most folks think they know what it means to know stuff. They say that you know X just in case you think X, X is true, and you have good grounds to think X.
That can't be right. Let me show you.
Let's say your boss says lots of times, "I will pick Jones for the job." You see that Jones is six feet tall. So, you think that that the dude who will get the job is six feet tall. You have good grounds to think that, since your boss said that Jones would get the job and since you can see how tall he is.
But then it turns out your boss picks you for the job, not Jones. You are six feet tall, too, just like Jones.
So, in this case, you thought that the guy who would get the job is six feet tall. It is in fact true that the guy who would get the job is six feet tall. You had good grounds to think that, too. But you did not know it. This means that what it is to know X is just not just for X to be true, for you to think X, and for you to have good grounds to think X.