A TAS, or Tool-Assisted Speedrun, is a series of inputs representing button presses that is made to demonstrate perfect or near-perfect gameplay. TASes will often use strategies that would be impossible or near impossible for humans to perform in real time. To make this TAS, I had to manually edit which buttons were and then check to see if it did what I wanted it to.
The purpose of a TAS is just to show what could be possible without human limitations, and not to pass off as or compete with human gameplay.
So what makes a game good for TASing?
Well, an important part is how the game is programmed. If the game isn't tick-based, or not everything runs on the same clock, it will be unstable. The game will not run the exact same way every time, and so making a TAS would be impossible without editing the game.
In terms of making an interesting TAS, games with fast or advanced movement, as well as strange glitches seem to work well.