This option specifies the shell configured for mailbox users. A mailbox user is the least privileged type of user, and normally should have very restricted access. However, if your Virtualmin server is only used by trusted users, or users that must have shell access, you could alter this from the default of /dev/null to one of the real system shells. Note that /bin/false is a special shell that will close remote SSH logins but will still allow FTP logins.