If you don't have a cable tv set top box,
yet you do have cable tv signal
... the QAM tuner tunes the digital cable signals.

Quadrature amplitude modulation (QAM) is a modulation scheme which conveys data by changing (modulating) the amplitude of two carrier waves. These two waves, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriersÂhence the name of the scheme.