Basically, it's using a program to silently (unnoticeable) install software (in this case, PPI software).
Not going to go into how to program them since it's most likely not allowed here (it used to be I think, but not sure about that now since I think silent installers were part of the reason the section got removed from BHW some time ago, wasn't around back then so can't say for sure -- that or people using PPI with things like SSM).