For a filesystem to be available to users, it must be mounted. The root and other filesystems in /etc/vfstab are always mounted as part of the boot procedure. The mount command that makes these filesystems available is run by startup shell procedures.
The mount command causes the mounted disk device and the mounted-on directory (the ``mount point'') to be associated with certain other information in the /etc/mnttab file. This information includes data such as the filesystem type, the mount options used during the mount, and the time the mount was performed. See mnttab(4).