What does over/under mean?

A bet on a total. The market sets a line, and you pick whether the real number lands over or under it.

Total points in the game, total goals, total tweets: the book sets the line where it thinks the crowd will split 50/50, and you pick a side. You're not betting on who wins, you're betting on how much of something happens.

Prediction markets ask the same question with the mask off: "Will Bitcoin be above $65,000 on August 31?" is an over/under where the percent tells you exactly how likely "over" is. A sportsbook hides that number inside an even-money line; here it's the headline.