I'll also say that the phrase doesn't have a widespread fixed meaning.
Based on your description, he/she is most likely making space for you to bring up anything at all you choose. If your boss has a work related question, it will be precise, directed and unambiguous.
- The vague question transfers control over the subject of your response to you fully.
- The rhyme is a clue that it's a less formal moment and an invitation to be creative.
What are you going to do with a vague question?
You could talk about sports, weather, a gripe, something that interests you like family or current events. If you are unsure, just answer "All's good, thanks for asking." but hours or days later turn the tables and ask the same question back.
Watch closely as your boss reacts.
I'd encourage you bring up anything that's safe and comfortable - have a little fun and see how your boss reacts. No matter what you say, within reason, it will be correct since it will after all, be... "the word, bird."