What does the idiom sitting on top of the world mean?

It means you are sitting pretty, living the life of Riley. It means everything is going your way professionally and personally. Usually said after some good luck or fortuitous happening.

When you are "on top of the world," you are feeling great. Example: "What a beautiful day, today. I am on top of the world!" Sometimes you are "on top of the world" because a great thing happened to you; other times it is just because you are feeling great. Example: "What a great time we had that night; we were on top of the world!" If you climbed the highest mountain and looked out to see a beautiful view, you would feel great and be on top of the world.

Everything is great. Everything's going your way. You're riding high. You're in clover. Life couldn't be better. You're a winner. You've got it made.

It means your the happiest you could be in your life!

it means when you are doing good in something