Sound absorption is when sound is absorbed by a material instead of bouncing back. Sound absorption is used to eliminate echoes, reverberation, and improve sound quality within a room.
NRC, noise reduction coefficient, measures how much sound a material can absorb. If you don't want to block sound, but absorb it, look for materials with NRC ratings as close to 1 as possible.
Sound absorption does not block sound transmission, see soundproofing.