1.
on, over or covering sth/sb :
Books were piled on top of one another.
Many people were crushed when the building collapsed on top of them.
2.
in addition to sth :
He gets commission on top of his salary.
On top of everything else, my car's been stolen.
3.
very close to sth/sb :
We were all living on top of each other in that tiny apartment.
4.
in control of a situation :
Do you think he's really on top of his job?