It's a popular GUI for stable diffusion models with many extensions. Like the sibling comment points out, everyone calls it like that because that's the handle of the original maintainer. (As in: Which web ui? Auto11's.)

Its a GitHub username https://github.com/AUTOMATIC1111/stable-diffusion-webui

I try explaining it here https://github.com/beothorn/OneClickStableDifusionAutomatic1...