Jacks or Better is not slang but a game: the classic single-hand video poker variant that most other video poker titles build on. Video poker has enjoyed a revival with the growth of online casinos, and game studios now offer many versions. The rules are simple. You are dealt five cards, choose which to hold, draw replacements once, and the final hand pays according to the paytable.
The name gives the minimum winning hand: a pair of jacks or higher, meaning jacks, queens, kings or aces. Pairs from twos to tens pay nothing. Above that, the usual poker ranking applies: two pair, three of a kind, straight, flush, full house, four of a kind, straight flush and royal flush. Paytables are the main difference between versions. The well-known “9/6” table pays 9 for a full house and 6 for a flush per coin, and returns about 99.5% with optimal strategy.