There are 3 parts to that. The first part says "if the disk has errors mount it read only (ro). The second part ( the 5 field in fstab) says
Originally Posted by unibroker
The third part (the 6th field in fstab) says
The fifth field, (fs_freq), is used for these filesystems by the
dump(8) command to determine which filesystems need to be dumped. If
the fifth field is not present, a value of zero is returned and dump
will assume that the filesystem does not need to be dumped.
The sixth field, (fs_passno), is used by the fsck(8) program to deter‐
mine 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.