What is a User-Moderated Forum on the Internet?
A user-moderated forum is a forum where users can become moderators, as opposed to moderators always being part of the staff of the website. What this means exactly depends on how it's implemented, but it typically means the moderator is a unpaid volunteer.
On some forums, the administrator may get in contact with active members in their community to grant them the role of moderator, and this may involve some required training and code of conduct, responsibilities that the moderator must uphold.
Meanwhile, on some platforms like Reddit and its countless clones, a moderator is just somebody. Anyone can create their own forum on such platforms and on doing so they become its moderator. They may have no training or capacity whatsoever to moderate anything, just as you would expect from any random user, but in this case they are also not held accountable for their poor moderation. After all, if someone creates a forum and moderates it badly because they're just a user, and you remove that role from them, who is going to moderate the forum afterwards? Just another random user who is probably going to moderate it badly as well.