Noise reduction headphones are headphones which are designed to reduce ambient noise (noise generated by the environment).
They work in two ways.
The materials of the headphones themselves (high density foam or other sound-absorbing materials) block out some sound waves, especially those at higher frequencies.
They also add an extra level of noise reduction by actively erasing lower-frequency sound waves. They do this by creating their own sound waves that mimic the incoming noise in every respect except one: the headphone's sound waves are 180 degrees out of phase with the intruding waves. This has the effect of reducing the incoming noise.