rpm --rebuild: what does it do?

[Stuart McGraw]

Hello all,

The man page for "rpm --rebuild", with rather excessive 
terseness, says:

> [...] use --rebuilddb to rebuild the database indices
> from the installed package headers.

I want to backup the "rpm database".  Presumably I
do not need to backup the "database indices" since
they are recreated by rpm --rebuild.

Can some translate "rpm database" and "database indices"
into filenames?  Thanks.

[P.S., I quickly scanned the Max RPM book, especially
chapters 6 and 7 and did not find an answer there though
it is a long book and I may have missed it.]