From the man page:
Define the behavior when an error is encountered. (Either
ignore errors and just mark the filesystem erroneous and
continue, or remount the filesystem read-only, or panic and halt
the system.) The default is set in the filesystem superblock,
and can be changed using tune2fs(8).
Essentially, when there is an error mounting the disk with read/write privileges, it will instead mount it as 'read only'.
Whether or not 'defaults' are used in your setup are not necessarily relevant. Yes, you can expect 'defaults' here, but the lack therein doesn't hurt you a ton.
As well, the '0 1' part isn't part of the mount options - everything in an fstab line is space-delimited / tab-delimited - any space between things indicates the 'next argument' to use.
As a security person, though, my advice is that if you believe your system was tampered with during installation outside that of the typical installer, then you should not be using that system, and should install with a known, good ISO and no networking during that installation to rule out network boot installer images causing issues.