What is square law device?

A square law device is something where either current or voltage depends on the square of the other.

For example, a saturated MOSFET has I proportional to V^2.

A diode's characteristics also has a square but it's not as clean. It's exponential, and through a Taylor series you get linear, square, cubic terms....

Anyway, it's useful because you can use it as a modulator or mixer.

Adding voltages is easy (just use resistors or an op amp) but multiplying is hard. With a square law device like a MOSFET you can do this:

Now if you follow that with a bandpass filter centered on you just get

Amplitude modulation.