I don't think anyone had a go at answering this yet? I'll have a crack.
So basically, an AI (Artificial Intelligence) 'chatbot' or 'bot' is a program that simulates human conversations using NLP (natural language processing).
Normally they are designed to act as an interface between businesses and their customers on websites (British Gas anyone???!) and vary in effectiveness.
These 'bot' programmes can be tweaked and deployed to act as fake posters on chat boards, Twitter, Facebook - anywhere that's free to use attracts them, basically. Elon Musk knew/knows that there are huge numbers of them on Twitter.
A clever bot owner might occasionally take over from the bot to make a few sensible and personal posts, to keep up the charade more convincingly. These hybrid 'botsters' get paid but very poorly.
It's very very cheap way to keep repeating a message, whether it be political propaganda, celebrity twaddle or erm, other things.