Preemptive Raises apply when you are bidding in a competive auction and your side has a know fit.  Your partner opens a suit and your RHO overcalls. Playing preemptive raises you would jump in your partner's suit to show a weak hand with 4-card support.  Your bid makes it more difficult for the opponents to compete effectively.  This bid is based on the Law of Total Tricks. In theory, you are safe to bid to the combined strength of your fit so with a 9-card fit it is usually safe to bid to the 3 level.

If you use preemptive raises you also need to use cuebid limit raises to show those values.   Therefore, if the bidding were to go 1H by your partner and 1S by your RHO, a bid of 2S by you would show a limit raise in Hearts.   Conversely, a 3H bid by you would show a weak hand with good support.

North opens the bidding 1H.  East overcalls 1S. This partnership has agreed to play Preemptive Raises.

With a minimum hand and 4 trumps North jumps to 3H.  If North had a true limit raise for his partner in hearts he would cuebid the opponents' suit to show this.  His jump to 3H is preemptive.