Mini comics were small xeroxed comics available by mail from the creators.
At one time there was a huge network of fans & artists who'd trade and sell these little limited editions through ads in such publications as the Comic Buyers Guide and Factsheet 5 (which was a huge magazine of nothing but reviews and ads for self published stuff.)

This one with art by Bill Fitts, Mark Bode and Eric Talbot demonstrate what folks used to do on long flights before the portable electronics became commonplace. Bored artists would draw, on anything that was handy.
I'll print a sample from the comic next week and see what other mini comics I still have.