What does apache module mod_auth_passthrough is for exactly? It seems to be shipped with CPanel and sometimes is active. What exactly is it for and what does it do?
mod_auth_passthrough is an Apache module that is used to allow other programs to use their own authentication methods and passthrough the Apache webhost. (Pass-Through-Authentication)
cPanel uses it specifically to allow FrontPage® to use its own authentication system.
For more information on passthrough authentication see here
I can't find the source file of the module currently (mod_auth_passthrough.c), but reading that would provide a more detailed explanation of how it the module implements pass through authentication.