Mysteriously they edited one of my questions
A bot that does various automatic tasks on the forum (set up by Discourse)
It edits your posts if you have an image url from an external source (if you didn't save it to your device). It takes the url and converts it to one that is supported by Discourse's database.
It also closes topics if they have a lot of flags.
It also does other things I don't remember.
System is just the robot that controls life here on the forum. They probably didn't change your post, but if your post contained a gif/photo, it might have edited it to save it to the database.
Overall, it's nothing to worry about. Just the forum bot doing it's job!