Sure! This joke is a play on words that involves a few different concepts.

1. The opossum is the main subject of the joke, and it's an animal that's not typically associated with being an accountant or working in a bank. It's an unusual and unexpected choice for the joke.

2. The word "bussing" is the key to understanding the punchline. The word "bussing" can mean either riding a bus or hitting someone with a fist. In this case, the joke is using the second meaning of "bussing" to create a humorous double entendre.

3. The punchline is a wordplay on the phrase "only one who knows how to balance the books." This phrase is used to describe someone who is good at managing their finances or handling a bank account. In the joke, the opossum is the only one who knows how to balance the books, which means they're the only one qualified to work as a bank accountant.

4. Finally, the punchline takes advantage of the double meaning of "bussing" to create a humorous contrast between the two ideas. The opossum refuses to take the bus because they're the only one who knows how to balance the books, but the joke implies that the opossum is so good at their job that they don't need to "buss" (hit) anyone else to prove it.