What is "bass trap"

n. A device, usually a foam block, placed in the corner of a control room or listening room in order to reduce unwanted low-frequency noises.

Bass Traps are acoustic energy absorbers which are designed to damp low frequency sound energy with the goal of attaining a flatter low frequency (LF) room response by reducing LF resonances in rooms. They are commonly used in recording studios, mastering rooms, home theatres and other rooms built to provide a critical listening environment. Like all acoustically absorptive devices, they function by turning sound energy into heat through friction.