1. To "Bite off more than you can chew," means to take on more than you can handle, or taking on responsibility that is just too much for you. 2. "Pardon the pun" is acknowledging a pun is in what is said, and is a way of asking (jokingly) for someone to ignore it. The asker expects that people will react, and it is a way to acknowledge that it is there, and then get back onto the main point. In this case, the python is swallowing a crocodile, which seems to be something that is too big for the croc to handle. In acknowledgement, the speaker says it looks like he has bitten off more than he can chew. Since, however, both the metaphor of "Bite off more than he can chew" and the current action of the python involve eating, and the python may actually have more than he can swallow, a very weak pun (a joke that is a play on word sounds or meanings) is made. the speaker just acknowledges that the pun is there and is moving on with the speech.

A pun is a play with words or A joke using a word or phrase that has another meaning or sounds similar to another word