1. informed about and in control of a situation, in addition to something
be on top of things at work
To be 'on top of' something can mean to be on the highest point or to cover something. 'On top of' can also mean to be in command or in control of a situation, like being 'on top of your work'. It can also mean 'in addition to'. "On top of all this work, I also have an exam to study for tomorrow." Or it can mean to be in very close proximity physically to other people. "The office was so small, we were all on top of each other."