Total Tricks Bids
The "Total Tricks" theory says that in competitive bidding, it is safe to bid up to the total number of trumps held by both sides in their best suits.
Without going into all the details (get Larry Cohen's excellent books on the subject), it turns out that an element of Total Tricks is that it is safe to bid to a level equal to the total number of trumps you and your partner have, even if you don't have ANY points.
If your partner opens 1S and you have 4 Spades, you have at least 9 Spades (assuming you only open a major with at least 5 of the suit), so you could preemptively raise him to 3S (9 tricks) even if you have no points.
In short, the theory is that if you are so weak that you go set at 3S when you have a 9-card fit, then the opponents have enough points to have scored more by bidding their own suit. Also, it is harder for your opponents to double your contract when they don't hold many of your trumps, so you can often make such a bid undoubled.
Here is another example to show an extreme case: your partner opens a weak 2H, promising 6 Hearts. You also have a weak hand and 6 Hearts. With 12 trumps between you, you can jump to 6H. There is a good chance that your opponents have slam somewhere, but how are they going to find it now?
Note that for TT bidding to work requires disciplined bidding by both you and your partner in competitive bidding situations. For example, a weak-2 opening is supposed to show a 6-card suit, but many experts will open a weak-2 with anywhere from 5 to 7 in a suit, depending on the situation. If you have 3 of partner's suit, you have to assume that partner has the minimum (5) and thus you can only compete safely to the 2 level (8 tricks). But if you always open weak-2's with 6 of a suit, then you could compete safely to the 3 level. So while the flexibility of opening a weak-2 with 5 of a suit on occassion may have its advantages, it clearly also has its disadvantages. The same is true if you sometimes open 1S with a 4-card suit when playing "5-Card Majors", or if you overcall with a 4-card suit (most people require at least 5), or if you preempt on the 3 level with a 6-card suit, etc.