Passive inter modulation, abbreviated as PIM, is an unwanted side effect mostly found in multichannel TX/RX systems, like for example telecom base stations and telecom satellites. When in the close vicinity of the TX antennas, metallic structures are found with different types of material, PIM is one of the effects that can occur.
How does it work? When you have a junction of two different conducting materials like for example copper and iron, the junction where these two materials meet a diode like electronic route exists between the two materials. A diode is a Non-Linear element in electronics.
When this metal junction is close to the TX antennas, RF currents will be induced in these conductors on all the active transmit frequencies. Because of the non-linearity of the 'diode' like junction, these frequencies will inter modulate on the junction and the produces products can fall into the RX channels of the system. If this happens, then the system is creating interference on its own RX channels, reducing sensitivity or in the worst case completely blocking the channel.