The waxing refers to the faze of the moon when the glowing part grows slowly larger over time. If one were to wax poetic, they would be speaking in a poetic, flamboyant, musing or creative way, and going on and on with it.
In this instance the word "wax" means to grow or become. So this phrase means you are growing poetic or speaking poetically. Poetic speech is not limited to Dr. Suess or other rhymes! It can be a lyrical or musical way of describing something or someone. Or it could be referring to a nicely descriptive phrase.
