It is usually f1+f2, f1-f2, 2f1-f2, 2f2-f1, 3f1-2f2, 3f2-2f1, and more sums and differences of the two frequencies and their harmonics. The higher the order, the weaker the intermodulation product.
On musical instruments, it is the beat note produced when two other notes are produced. On a radio, if 2 strong stations are there, they will appear in the wrong places on the dial on cheap radios.