With respect to loudspeakers, the back wave is the sound emitted from the back of the loudspeaker drivers. The backwave can cause cancellation of the (desired) front wave of the speakers. This is mostly an issue at low frequencies, as deep, long-wavelength sounds bend around objects more than high, short-wavelength sounds.

If it were not for the backwave, we wouldn't need loudspeaker enclosures - their main function is to do something about this sound cancellation. Some enclosure types (acoustic suspension), absorb or minimise the backwave whilst others (bass reflex) utilise it to some degree.