btrfs subvolumes: What is @rootfs and @?

In the video [_"Debian 12 Bookworm Minimal Install w/BTRFS "_ by JustAGuy Linux at 8:28](https://youtu.be/MoWApyUb5w8?t=494) you see the author run

mv @rootfs @

> You see that @rootfs that is the subvolume that's been created but we're going to move that specifically for [timeshift](https://github.com/linuxmint/timeshift) . And we're going to say move the @rootfs to just the at symbol (@). [...] We're going to create a second subvolume we're going to say btrfs subvolume create @home I don't understand this though, * Why was the original subvolume created by the Debian installer called @rootfs? * Why does timeshift not like this, or does Timeshift require this? * Is there anything special about a subvolume named @?