A ‘Baby Flush’ is a lower valued flush, that has the potential to be beaten by a bigger flush. A flush is five cards of the same suit in any one hand. The best possible flush is the ace high flush, for example A-4-6-8-9, all of the same suit. The term Baby Flush is used when a player hits a smaller flush (not the ace high), for example a 9 high flush (9 being the highest card of the 5 used to make the flush).
This is always the risk in playing both rag suited cards or lower value face cards. When you hit your flush, it’s often hard to lay the hand down and a player holding the ace high flush can do significant damage (i think every good player has been there!).