The ear behaves as if it contains a bank of filters, each filter passing frequencies within the critical band. Two tones seperated by more than a critical bandwidth are recieved by the cochlea more-or-less independently. In human hearing, only those frequency components within a narrow band,called the critical band, will mask a given tone. Critical bandwidth varies with frequency between 1/6 and 1/3 octave.
Which sounds are audible, and which are masked (hidden) by other sounds can be better understood by working with critical bands. For this reason, many sound quality metrics begin by putting the measured spectrum into critical bandwidths.
The Bark is the standard unit corresponding to one critical bandwidth of human hearing.