Foreplay has traditionally been used to refer the sexual and intimate acts that occur in preparation for intercourse. In more recent years, however, this definition has been rejected in favor of a more inclusive one that favors more warmup and slower sex, no matter what kind of sex you're having (because not everyone has intercourse!)
Foreplay increases arousal and desire between partners. It also helps to lower inhibitions and increase emotional intimacy. Kissing, touching, oral sex, massage, and other sexual acts that don't bring either partner to climax may be considered part of foreplay.