Plain English Meaning
How well the brakes will actually stop the airplane on the runway you are using. Good runway and dry tires mean the brakes work well. A wet, icy, or contaminated runway means the brakes work less well, no matter how hard you press the pedals.
Full Definition
The degree to which an airplane's wheel brakes can actually slow the airplane during the landing roll, determined by the friction available between the tires and the runway surface. Braking effectivity is reduced by water, snow, ice, slush, rubber deposits, or any condition that lowers tire-to-runway friction, and it depends on sufficient being on the wheels for the brakes to bite.