in macOS, a guest account allows a user to login with the username of guest, without a password. When logging out, anything created by the guest is deleted and the computer returned to its pre-guest status.
The “guest” user in current generations of macOS is built into the OS and cannot be removed.
However, that doesn’t mean you need to have it active. You can disable the guest user account so that it can’t log into the computer nor file share with it.
Most IT security pros would say to keep the guest user login on. Should the computer get stolen (or lost), this allows someone to log in to the computer, and trigger Find My Mac. Now you can see where it is at. With guest off, nobody can login, and you can’t find your Mac.