TAS is short for tool assisted speedrun. It's used to create a theoretically perfect run. You can slow down time, see more information about your car, and send direct inputs to control the car, as well as other things. A person needs to create each run, but it is tool assisted, hence the name.
Thanks for the explanation Byzonyx.
My own opinion for what it's worth is if the player makes it clear that the run was computer assisted by using a profile name such as Wizard [TAS], then that is OK, the "perfect" run can be interesting, Otherwise it is just plain cheating, and cheating is for losers.