Explaining what TASing is to people outside of the community, especially outside of speedrunning, can be a huge challenge. The following is my typical progression for walking a person through what this niche is about without overwhelming them or getting too far off track. Feel free to reuse/repurpose this as you wish.

The basic premise is that each section is built on top of the last. In my experience, this approach works really well as it limits what a person needs to understand before moving to the next, more advanced, concept.

Do you know what video games are? (Should be a yes for everyone. This question is to set the context of the discussion so that the recipient has the right mindset going forward.)

There is a niche within gaming called speedrunning. People who speedrun games attempt to complete a game's objectives as fast as humanly possible, such as beating the final boss or collecting all items. They may end up replaying the game thousands, if not tens of thousands of times to achieve a fast time.

There is an even smaller niche withing speedrunning called TASing, or, tool-assisted speedrunning. People in this niche use tools to craft a theoretically perfect speedrun. These tools enable a player to step through a video game one frame at a time, and decide precisely which buttons should be pressed on any given frame. The result is a carefully selected sequence of button presses that, when replayed in real time, play the game well beyond what a human would normally be capable of.

Some notes and additional comments you can add: