If your team takes enough tricks that the opponent cannot make their bid, you have "set" them. (You can also say that you "set" a nil bid by playing your cards right... on this page, I am talking about setting a bid that is not zero, however.)
If the sum of all bids is greater than 13, you must aim to set your opponents, since one side or the other is going to get set, by definition. For example, if your team's total bid is 3, and the opponents' is 11, then the grand total is 14. You will cause the opponents to lose 110 points simply by taking your 3 tricks.
If the total bid is 13, you should also try to take as many tricks as possible. Your opponents will almost certainly be doing this. Just taking one extra trick is enough to do significant damage to the opponents.
Below is a video clip that shows an example of this. The total bid is 13. I was a little bit daring in my 3 bid, but I managed to take all 3 tricks. My partner was not lucky, and we lost 60 points.
Here's a replay of that hand: My partner counted on the Ace and King of hearts, and the Ace of clubs. He is a bit "long" in clubs and hearts, so counting all those as taking a trick was risky. In addition, he has only one trump - the 2 of spades. With 12 non-trump cards, he will almost certainly never get a chance to trump a trick.
You can put all the blame on Robby for our set, but I don't. This situation is also my fault. I was the last to bid. I knew that pushing the bid to 13 would put us at risk of getting set. I did take all the tricks that I counted on, but I did not have a very strong hand. If I had been a little more cautious in my bid, things would have gone differently. We were both a little bit too ambitious, and this caused the set.