What exactly is the under/over 2.5 goals market then?

It's a market where you bet on the number of goals in a football match - in other words, whether there will be more or fewer than 2.5 goals.

That makes sense, but why 2.5 goals?

A team can't score 2.5 goals! Exactly - by setting the 'goalline' (as it's known) at 2.5 goals, the bookies allow you to take a view on every possible scoring outcome. You canbet on whether there will be two goals, three goals, or so on. Remember, you're not betting on there being 2.5 goals in the game, you're betting on more or fewer than 2.5 goals.