Btrfs is a Linux copy-on-write filesystem with subvolumes, snapshots, checksums, compression, and send/receive. A subvolume is another filesystem root inside the same Btrfs filesystem; it is not a separate partition. The kernel’s Btrfs documentation lays out the available features.
A snapshot gives you another view of a subvolume at a particular moment. At first, both views share unchanged data. When one changes, Btrfs writes the changed extent separately. That is why snapshots can be quick to make and cheap to keep for a while. Retain too many, or change a lot of data, and they start using real space.
A snapshot is good insurance before a risky package update or configuration change. It sits on the same filesystem as the original, though, so a dead drive can take both with it. Keep a separate backup for files you care about, and test restoring it. Snapshots and backups cover different failure cases.
Subvolume layout matters too. /, /home, caches, and mounted data may live in different subvolumes. A snapshot of root will not magically capture a separate home mount. Boot-time rollback adds another link in the chain: the snapshot tool, bootloader, mount layout, and retention policy need to agree.
A safe way to learn is to make a disposable subvolume, snapshot it, change a file, compare the two views, and restore the test. Then look at the actual mount layout before adopting a desktop guide’s commands. A line that assumes @ and @home can be wrong on a system laid out differently.
Btrfs gives useful building blocks. The operator still chooses what to snapshot, how long to keep it, and where the second copy lives.