caliper slide?

What is a brake caliper slide? I have only done brakes where i didnt have to rebuild the caliper. Im about to do work on my van after i had brakes done on my car. They charged me under labor for cleaning a lube of the brake caliper slides. How do you do that?

I'm not sure what they do but I know I had to replace them the last time I did the brakes on my camaro too.