Most, if not all file systems, assign a universally unique identifier (UUID) which looks like a random hexadecimal string to each file system. On a Gentoo system, you can get an overview of all UUIDs detected using a simple ls -l /dev/disk/by-uuid, courtesy of the gentoo udev rules. Users can also assign a specific label to a file system, either when they create it (like mkfs.ext4 -L ROOT /dev/sda2) or afterwards (e2label /dev/sda2 ROOT). This is also not limited to the content file systems - you can also assign a label to a swap file system.
This information can then be used to uniquely identify the file system, even if you don't know what the device file (/dev/sda2) is called. A huge advantage is for those devices that often change device file (removable media, but also SATA or SCSI disks on systems where the admin loves adding and removing disks ;-) as you can keep your fstab configuration static: the fstab file doesn't need to be changed, even when the device files themselves change.
A simple fstab line (
/dev/sda2 / ext4 defaults,noatime 0 0) can
then easily be transformed to use the LABEL="..." syntax (like
LABEL=ROOT / ext4 defaults,noatime 0 0).
Some people might also think they can use this mechanism for their
kernel boot parameter (like using
root=LABEL=ROOT instead of
root=/dev/sda2). Sadly, the Linux kernel doesn't offer this
functionality. It is possible, but only when you use an initramfs (which