When gambling on an event whether it be a card game, at a casino, at a race track, etc., two or more people may choose to place a seperate bet away from the main pool of betting.
Lets say Joe, Bob, Chris, Tom, and Becky are playing a card game and they are all betting $10 a hand. Tom and Joe may get a little extra competitive and decide to place a side bet between just the two of them. They will bet the originally agreed upon $10 on a game with everyone at the card table involved and they will bet, lets say another $25 just between the two of them.
Someone you keep on the side even though you have an interest in others.
I'm just your sidebet.