Explanation
The command mount -a mounts all filesystems that are listed in the file /etc/fstab and have the option auto set.
The option auto means that the filesystem can be mounted automatically by the mount -a command or at boot time. The option noauto means that the filesystem can only be mounted explicitly by the mount command with the device or mount point specified12. The file /etc/fstab containsinformation about the filesystems that are known to the system and how they can be mounted3. The command mount -a is useful for mounting all the filesystems that are needed by the system after a change in /etc/fstab or after a reboot4. References: 1:
mount(8) - Linux man page 2: Linux mount and umount commands help and examples 3: fstab(5) - Linux man page 4: How to Use the mount Command on Linux