sudo btrfs subvolume list /
https://wiki.archlinux.org/index.php/Btrfs
"A btrfs subvolume is not a block device (and cannot be treated as one) instead, a btrfs subvolume can be thought of as a POSIX file namespace. This namespace can be accessed via the top-level subvolume of the filesystem, or it can be mounted in its own right." [5]
Each Btrfs file system has a top-level subvolume with ID 5. It can be mounted as / (by default), or another subvolume can be mounted instead. Subvolumes can be moved around in the filesystem and are rather identified by their id than their path.
See the following links for more details:
Btrfs Wiki SysadminGuide#Subvolumes
Btrfs Wiki Getting started#Basic Filesystem Commands
Btrfs Wiki Trees
Creating a subvolume
To create a subvolume:
btrfs subvolume create /path/to/subvolume
Listing subvolumes
To see a list of current subvolumes and their ids under path:
btrfs subvolume list -p path
Deleting a subvolume
To delete a subvolume:
btrfs subvolume delete /path/to/subvolume