The point system in chess gives a rough indication of how strong each piece is. So the short answer to your question is, yes, a rook is more valuable than a knight, and so in the vast majority of cases, if you can trade a rook for a knight, you should do it.
This does beg the question, however, about why a rook is considered more valuable. At its best, a rook can control long files (columns) and ranks (rows). While the knight's tricky movement can sometimes be an advantage, in general we consider it to be inferior. One way to demonstrate the value of a rook is to try the rook and king vs king endgame. (I would definitely recommend looking it up first.) Note that there is no way of checkmating just with a knight and king. In fact, the game would be drawn by insufficient material. This is just a fancy way of saying that neither player has enough firepower to achieve checkmate. As you progress, it will become clearer to you why rooks are more valuable than knights. At some point, you might even be able to notice when a knight is better than a rook! But for now, concentrate on the points system—it's a great way of knowing who is ahead in the game.
Edit: it's worth noting that the points system is not part of the rules of chess. The bottom line is that you are trying to checkmate your opponent's king. But having more material (i.e. points) is a key way of getting closer to this goal.