To be more specific is about testing the Cross Modulation, which is a type of interference where the carrier of a desired signal becomes modulated by an undesired signal situated on a different carrier frequency.
In simple terms, a large off channel signal , but within RF stages bandwidth, with AM modulation, drives the smaller desired signal up and down the slightly non- linear gain slope of the receiver's active stages. Because of the gain slope variation, the desired signal is modulated with some of the undesired signal's AM content.