This is usually a bet made with a friend or different bookmaker in addition to the straight bet placed in a sportsbook. Often times, a side bet is a prop bet and is unrelated to point spreads or winning and losing.
A side bet can be just another name for a wager or any other type of bet. Its usage is dependent on context. When speaking with Bookmaker A, a side bet may be the money line bet you made with Bookmaker B, while when speaking to Bookmaker B, a side bet is the bet you place with Bookmaker A. Making bets on the side is a popular part of gambling culture. Since most professional gamblers will bet on almost anything in sports, bets between gamblers on who will have a bigger day (profit), which line is right, or even as simple as who will win the game, are very commonplace.