A floating brake caliper has to slide sideways to press both pads evenly against the rotor. The parts that let it slide are the guide pins. Here is what a brake caliper guide pin is.
What it does
Guide pins (also called slide pins) are the smooth bolts the caliper slides on. When you brake, the piston pushes the inner pad while the caliper slides on the pins to pull the outer pad in - centering the caliper so both pads share the load; see caliper slide pins.
Why they must move freely
If a guide pin seizes from dried grease or corrosion, the caliper cannot center, so one pad drags or wears far more than the other; see why calipers stick. Free-moving pins are essential for even braking and long pad life.
Boots and grease
Each pin rides in a rubber boot that keeps grit out and grease in. A torn boot lets the pin corrode and seize; see which grease where. Fresh high-temp grease and intact boots keep them sliding.
Service them
Clean, inspect, and re-grease the guide pins at every brake job, and replace pins or boots that are worn; a semi-loaded caliper brings fresh ones. Keep braking even with quality pads and hardware.