What does it mean be there by 5 O'clock?

Answer 1

Five o'clock is the time at which you are expected to arrive at the meeting or party or whatever it is.

Answer 2

It means that you are required to be at the designated location shortly before either 5:00 am or 5:00 pm (which of the two should be clear from the context).

Answer 3

O'clock means "of the clock." They're telling you a time. You should be there before 5:00 - but you should ask if they mean in the morning or in the evening!