BtrFS (B-tree file system) is a new-ish kind of file system that, instead of writing a new file each time it changes, it will write a "base" file, and then store changes made to that file, kind of like a diff. This approach (sometimes called copy-on-write, although that's a bit more complicated) optimizes the situation, where your system makes a lot of copies of the same file, for example on a server.
Whenever it makes sense, it will update the "base" file. This approach makes it real easy to roll your system back to a previous state, as you can just ignore every change made after the update. However, as file systems are a hell to test, it has been a long way since BtrFS was started until it became useful for end users.