fstab(5) man page:
The sixth field (fs_passno).
This field is used by the fsck(8) program to determine the order
in which filesystem checks are done at reboot time. The root
filesystem should be specified with a fs_passno of 1, and other
filesystems should have a fs_passno of 2. Filesystems within a
drive will be checked sequentially, but filesystems on different
drives will be checked at the same time to utilize parallelism
available in the hardware. If the sixth field is not present or
zero, a value of zero is returned and fsck will assume that the
filesystem does not need to be checked.