be there or be square

Be there or be square is a cliche referring to a party or special occasion or event in which anyone who has a certain type of social status or popularity will be at this special function. A square is the slang term for someone who is uncool, unpopular or unhip, otherwise a more simple type of person. So when speaking of be there or be square, if the person does not go to this function, they are considered to be uncool.