It means mount the partition /dev/hdb1 on the /mnt/windows mountpoint with type vfat. Mount it Read/Write (rw or just w), allow all users to mount/unmount it (users), give users all permissions (umask=000 -> take away permissions 000, that is take away nothing), do not dump the filesystem (zero before the last zero) and do not check it (last zero).
Explore
Code:
man fstab
man mount
As for losing any data, I'm not sure what you mean. You can't rely on the OS to police your users after you've allowed them complete control on a filesystem which is not even native to that OS