Question: Hey Bob and Rob! I have a question: would you explain the idiom be there or be square to me?

Response:
First, let’s look at an example of how this idiom can be used:

Rob: Hey, are you going to the party tomorrow?
Bob: Maybe.
Rob: Well, be there or be square.

A square is a person who is uncool or not hip. So Rob is telling Bob that if he is not at the party, then he will be considered uncool, or not in (not part of) on the action.

Interestingly enough, in the past square was also used in a positive way, to mean honest or fair. For example, square talk, a square fight, or a square deal.

You can learn more about the term square here.

~Bob