TL;DR: AUTOMATIC1111 is a username, not a software name. Still, many people post on the Web using it to refer to application software or other similar stuff, and some other users use this name together with the name of the corresponding item, application software, GitHub repository, etc.
The URL of the link included in the question is
https://github.com/AUTOMATIC1111/stable-diffusion-webui. This URL points to a GitHub repository.
github.com is the GitHub domain.
The part between the slashes (
/) after the GithHub domain, is the username or organization name; in this case,
AUTOMATIC1111 is a username.
The part after the second slash (
stable-diffusion-webui, is the repository name.
Snapshot of Automatic1111's user profile page at GitHub
Source: https://github.com/AUTOMATIC1111. Taken: August 13, 2023
Automatic1111 has 37 repositories on GitHub. As shown in the above image, several are related to Stable Diffusion.
For those unfamiliar with GitHub, this is one of the most famous spaces to share code using
git, "a distributed version control system that tracks changes in any set of computer files, usually used for coordinating work among programmers" ref. https://en.wikipedia.org/wiki/Git.
Among other things, GitHub allows people, most of them software developers, to share their open-source software development projects publicly. One of the GitHub characteristics that make it popular among non-experienced developers and non-developers is the possibility to use it without knowing about git, thanks to it's web user interface.
There is a similar question in Reddit: Who is Automatic1111?. The current top answer points to a Discord Server and mentions that
Automatic1111 is a real person who is using this as display name and user name on Discord.
level 1 KhaiNguyen·10 mo. ago
It's a real person, 1 person, you can find AUTOMATIC1111 in the Stable Diffusion official Discord under the same name. The AUTOMATIC1111 SD WebUI project is run by the same person; the project has contributions from various other developers too.