what does update /etc/fstab do?

Hi,

I am compiling 2.4.17UP version on my dual processor machine.  When I
reboot  with the new kernel, I just
can't pass the stage "update /etc/fstab". So I switch the reboot procedure
to interactive mode and bypass
update /etc/fstab stage.  Then I can boot the machine and log in and do
some work. Everything looks fine.
I have a few questions here:

1) What  does "update /etc/fstab" do?
2) What kind of harm will it cause if I skip this stage?
3) What can cause "update /etc.fstab" hang there?

Maybe all the questions are related. Thanks for any enlightenment.

hui