A "pun" that I can't understand, at all

In the book The Quantum Universe: Everything That Can Happen Does Happen by Brian Cox and Jeff Forshaw, on page 104 there seems to be a pun of authors:

Generally speaking, frequency is the fundamental quantity that describes standing waves, and this sentence is probably a pun.

No matter how many times I read this I can’t understand what the actual "pun" is. Is it because these particular waves are called "standing," whereas the word "frequency" implies something happening?