A win condition is the circumstance or sequence of events that your team (or the enemy team) needs to win the game. You play the map until you can execute your win condition, whether that be a 4 man Orianna ult, a pick on a priority enemy, forcing the enemy team to 5v5 if you have a 4 item hypercarry.

So, for example, your team has Fiora and their team has Sion in the top lane. They have an ADC like Kog and you have Ezreal or something. Your win condition is Fiora splitpushing so your decisionmaking should be around getting Fiora in a position where she can 1v1 Sion and push down turrets. In that situation you want to get vision control around baron and bait the enemy team in, blow Sion's teleport, and then force him to either group so you can stall while Fiora pushes, or leave him to 1v1 Fiora and lose anyway. You don't want to teamfight because you don't need to. It's not your win condition.