The term chalk in sports betting simply means that side of the bet is the favorite – usually a very heavy, or strong, favorite – to win that side of the bet. This can be a team, a player/individual, a statistical outcome or a horse.
Strong favorites are typically tied to very low odds, in the -300, -400, -500 variety. That means to win $100 on betting on that heavy favorite – or chalk play – you’d have to spend $300 (on -300 odds), $400 (on -400 odds) or $500 (on -500 odds) to have a return of $100 in winnings. Obviously that’s not a huge return on your investment.