Why When two RF signals are applied to a nonlinear device such as a mixer, the output contains new frequencies: the sum and the difference of the two inputs (plus the originals and harmonics). That process is called heterodyning, and it is the basis of superheterodyne receivers, where an incoming signal is mixed with a local oscillator to produce an intermediate frequency.
Watch out Synthesizing is tempting because frequency synthesizers do generate signals, often using mixers and dividers, but the term refers to creating a desired output frequency from a reference, not to the act of combining two signals.
Hetero = different, dyne = power: two different frequencies combine to make sum and difference.