An over/under (or "totals") bet is a wager on whether the combined score of both teams will go over or under a number set by the bookmaker.
If a sportsbook sets the total at 48.5 for an NFL game, you can bet on the over (combined score exceeds 48.5) or the under (combined score stays below 48.5). The half-point ensures there's no push.
Different bookmakers may offer slightly different total lines. One book may have 48.5 while another has 49. That half-point can flip a losing bet into a winning one. oddstatus compares totals across 9 books.