What is Raspbian Addons?

Raspbian Addons is a repository containing extra open-source software for Raspberry Pies.

What is an APT repository?

APT repositories are large collections of software packages in the form of .deb packages. In simpler terms, it's a remote directory, or server, from which your system downloads software from. Debian Linux, which is what Raspberry Pi OS uses as its base, uses APT as its system package manager.