The traditional concept of knowledge is a justified true belief.
Let's take an example. Suppose the Cubs win the World Series this year. A lot of fans are going to come out and say that they knew at the beginning of the season this would be the year the Cubs win the series.
First, the Cubs have to win the series in order for someone to know that the Cubs won the series. If the Angels win the series, no one can know that the Cubs won it.
Second, the person has to believe it. So someone who says he knew the Cubs would win would have to believe that the Cubs would win. Someone who was picking the Diamondbacks would not qualify as someone who knew that the Cubs would win.
Third, a person has to be justified in believing it. So the mere fact that one is a die-hard, incurably optimistic Cubs fan, who have predicted victory for the last 70 seasons would not qualify as someone who knew that the Cubs would win the series.
Some philosophers have posed problems with this definition (google the word Gettier if you want to know about that) but this is a fair enough rough idea of what knowledge is.