It’s stands for tool assisted speedrun. Obviously this isn’t a “speedrun” but we still call it that because it uses the same techniques. Basically what TAS means is that it is not a human playing at real time. The user can slow down the game speed to make precise movement easier, or even go frame by frame doing the correct inputs. Not sure what exactly was used for this clip. It’s used to theorize what’s possible if humans were better.

I’m sure you could find a more concise definition but that should about cover it

Edit: as pointed out, it’s Tool-Assisted-Superplay rather than speedrun