The other answers are really great, but I want to dig into the APGL a little.

The GPL said everyone you distribute your software to is also entitled to the code. This made sense in the pre-web-service era when software was some code you downloaded or got off a disk and ran on your own computer.

But with the internet it became common to run software on a server, which responded to distant users’ requests. But that was a problem for the GPL because now the software wasn’t distributed to the user, it was distributed to the server, which is controlled by me. So now I’m entitled to the code from my own changes, but the user has no such freedoms.

So the AGPL was created to try and close that loophole, and make it so that the internet users are protected as the license was originally intended to protect local users.