Over/Under is a bet on the total goals scored by both teams. The most common line is 2.5: three or more goals wins the Over, two or fewer wins the Under. Who actually wins the match is irrelevant.
Why the half-goal? It makes a push impossible — every bet settles.
What moves this market: pace of play, defensive schemes, and match state. Two attacking sides can still produce an Under if the first goal never comes and nobody has to chase; a cagey knockout tie can explode after a red card. Expected-goals (xG) numbers are usually a better guide than raw goal counts, because they strip out finishing luck on both ends.