“Bug” is poker slang for the joker, the extra card supplied with most standard decks alongside the 52 suited cards. In poker, the bug is a restricted wild card. Under the most common rule, it can only be used as an ace or to complete a straight or a flush; it cannot turn other cards into a pair or a set. In lowball games, it usually counts as the lowest card not already in the hand.
That is different from a fully wild joker, which can stand in for any card. Some video poker games, such as Joker Poker, use the joker that way, and their paytables are built around it.
Outside poker, jokers are removed for many card games but act as wild cards in others, including several rummy variants. Whenever a game uses a bug or joker, its exact powers should be stated in the rules, because they change hand probabilities and the payouts that make sense.