Block off plates are small, metal plates used to seal the ports on a motorcycle's cylinder head cover, replacing the Air Injection System (AIS) or PAIR system (Pair Air Injection Resistance). Primarily for racing or track use, they eliminate the smog system, reducing weight, cleaning up engine clutter, and stopping the annoying "popping" or backfiring noise in the exhaust during deceleration. 

This video demonstrates how to install block off plates on a motorcycle:

Function: They prevent the PAIR system from injecting fresh, oxygen-rich air into the hot exhaust stream, which is designed to burn off unburned hydrocarbons for emissions control.

Benefits:

Reduced Popping: Eliminates exhaust backfire and deceleration popping, particularly when using aftermarket exhausts.

Accurate Tuning: Prevents extra oxygen from skewing 02 sensor readings during dynamometer (dyno) tuning.

Weight Reduction: Allows for the removal of heavy hoses, valves, and components.

Usage: They are essential for racing or track-only applications to ensure a proper, accurate fuel-to-air ratio.

Legality: Often "for race track use only" and may not be legal for street use in some regions, notes this Driven Racing page.

Side Effects: While they stop popping, they do not directly increase horsepower, but are required to stop the air injection from interfering with accurate dyno tuning,