If you say that someone, for example, waxes lyrical or waxes indignant about a subject, you mean that they talk about it in an enthusiastic or indignant way.
He waxed lyrical about the skills and commitment of his employees. [VERB adjective]
Tom sat himself down and waxed eloquent about all the advantages of the new plan. [VERB adjective]
Synonyms: enthuse, rave [informal], gush, get carried away