If you say that you feel on top of the world, you are emphasizing that you feel extremely happy.

For example:

When she came back from that holiday, she was so happy, on top of the world.

She was feeling on top of the world after winning the tennis tournament.

I was on top of the world when I found out that I’d gotten an A on my hardest exam.

Cathy has been on top of the world ever since she got that big promotion at work.

Tradução:

O sentido literal da expressão on top of the world é “no topo do mundo”. Já o sentido figurado é “estar nas alturas” ou “estar em êxtase”.