The term is used in many card games if the rules provide for a special type of bets that are valid regardless of the principal amount at stake.
Side Bets – additional bets in gambling on which accumulative jackpots or bonus payouts are played.
For example, side bets are used in blackjack to drawing of the progressive jackpots.
In most of the additional rates laid pretty high mathematical superiority of the casino, so the theoretical return on they're too low. Accordingly, gambling experts do not recommend doing them.
About RTP in the side bets of different online games, read the reviews at Casinoz.