at the top of the hour: o que isso significa?

You have a class today very early in the morning and you're supposed to be there at exactly 6 a.m. You know how strict your teacher is and he hates people being late in his classes. So you go through your stuff, gets dressed in a second and while you're storming out through the door. your mother asks if you're not eating anything for breakfast before leaving. From the outside you yell: I can't. My class starts at the top of the hour. That means it starts at the exact beginning of an hour. If you spend time eating your breakfast, maybe there won't be time for you to be there at exact 6.a.m. That's all. Take care!