>>715512545It's a little bit like chess. The way you win chess is by capturing the opponent's king. However, chess is a game that requires thinking several turns ahead, and often it'll become clear that one player can't win well before their king's actually put in check. As a result, players will often concede early, recognizing that they've been put in a situation they just can't come back from.
Starcraft's similar. The victory condition is to destroy all of your opponent's buildings, but games rarely go that long. Instead, players generally concede when they recognize that they're toast. In the case of that game, the key units are those two invisible ones, called dark templar. As soon as the red player saw them, he understood that he had no way to kill them (or even see them). If the game had continued another thirty seconds, those two dark templar would have started butchering the resource-gathering units of the blue player like this Anon mentioned
>>715512952, which would have sent his economy into a spiral. To stay in the game, he'd have to spend money on some way to fend off the dark templar, while also recruiting more resource gathering units to replace the ones being killed, all while under attack by units he can't fight back against. Effectively, he'd been dealt a death blow and he knew it--despite no damage actually having been dealt yet. He could have played it out for a few more minutes, but it's considered both efficient and good sportsmanship to end the game when it's obvious that you're done for.
Side-note: those robots teleporting in actually belong to the blue player. That's just what it looks like when those units get recruited. However, they'd need some way to detect the dark templar in order to shoot at them, so they were effectively worthless to him in this situation. The blue player had a pretty respectable army actually, he just risked it all on the hope that he wouldn't need to worry about invisible units for another couple of minutes.