Stop and go or stop 'n' go is when a player bets into another player who has previously raised or otherwise shown aggression . Example: On the flop, Bill bets into Tom, Tom raises, and Bill just calls. On the turn, Bill bets into Tom again. Bill has just pulled a stop 'n' go play.
Another version of the "stop and go" is in tournament poker when a player raises pre-flop with the intention of going all in after the flop regardless of the cards that fall. This is typically done when the blinds are high and every chip becomes vital.