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.