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/etc/fstab

/etc/fstab is the filesystem table.
Each (uncommented) line has the following fields: 
/dev/sda5  none  swap  sw                        0 0
/dev/sda6  /     ext4  rw,relatime,data=ordered  0 1
/dev/sda7  /var  ext4  rw,relatime,data=ordered  0 2

Mounting all mentioned filesystems

mount -a mounts all filesystems mentioned in /etc/fstab. 
mount -a [-t fs-type] [-O optlist]
This command is usually used in a boot-script.
It's possible to combine mount -a with -F to make mount fork so that filesystems are mounted in parallel.

See also

mount, findmnt
swapon
genfstab helps to create an initial /etc/fstab file when a system is installed.

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