Sometimes it is called " Adjacent Channel Rejection Radio". In RF system, as you know, the envirement is very noisy and full of interference. The noise and the infererence from the working channel will effect the performance of the receiver greatly and the interference from adjacent channel can also affect the working channel through a phenomena call "reciprocal mixing". Then the ACRR means the ratio between the max adjacent channel interference, which the receiver can tolerence and the desired signal.