What is 'Other Volumes'?

A standard macOS startup APFS container does contain additional volumes to the volume with macOS installed on it. You can list these with diskutil apfs list. The standard configuration of such a container is as follows:

• disk1s1, the volume you boot from, mounted at /, shown in Disk Utility as Macintosh HD
• disk1s2, ‘Preboot’, not mounted, hidden
• disk1s3, ‘Recovery’, not mounted, hidden
• disk1s4, ‘VM’, mounted at /private/var/vm, hidden

The last 3 are grouped as Other Volumes in Disk Utility. They're required by macOS and shouldn't be removed.

While grg's answer is correct, I would like to point out that both the ‘Preboot’ and ‘Recovery’ volumes may contain files that are no longer needed. This occurs when a volume containing macOS stored in the APFS container is deleted. The files stored in the hidden ‘Preboot’ and ‘Recovery’ volumes, that are associated with deleted volume, may remain.

For example, when entering the command diskutil list on my Mac, I get the following output regarding an APFS container.

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +451.5 GB   disk1
                                 Physical Store disk0s3
   1:                APFS Volume Snapper                 291.9 GB   disk1s1
   2:                APFS Volume Preboot                 47.1 MB    disk1s2
   3:                APFS Volume Recovery                522.7 MB   disk1s3
   4:                APFS Volume VM                      8.6 GB     disk1s4

The ‘Snapper’ volume contains macOS Mojave. The the volume UUID for this volume can be determined by entering the command diskutil info disk1s1 | grep "Volume UUID". The output is shown below.

   Volume UUID:               47A8A135-5137-3651-90DB-5CE5C5D185C9

Entering the command diskutil mount disk1s2 produces the following output.

Volume Preboot on disk1s2 mounted

The command ls /Volumes/Preboot/ produces the following directories (folders).

47A8A135-5137-3651-90DB-5CE5C5D185C9    94C10540-E411-4520-B856-882C85E39002

Since the directory named 47A8A135-5137-3651-90DB-5CE5C5D185C9 matches the volume UUID of the ‘Snapper’ volume, this directory should not be deleted. However, the directory named 94C10540-E411-4520-B856-882C85E39002 does not match the volume UUID of any volume in the APFS container and therefore can be deleted.