Added for emphasis to a question,or to show you are surprised or annoyed.
ex: What on earth is coming on?
Something is very expensive.
ex:My necklace cost the earth.
An event or a new is extremely shocking and important.
ex:The president is dead,it is earth-shattering.
You have plenty of time
ex: Take it easy,you have all the time in the world
Something is so good and unbelievable
ex:she is so beautiful,She is out of this world.
Something is extending throughout the entire world
ex:A worldwide epidemic
when you have great affection and respect for someone
ex:I love my parents,I think the world of them.
When you feel absolutely fantastic.
ex: I am getting married,I am on top of the world.
The place where people dance.
ex: They are on the dance floor.
It is the land at the bottom of the sea.
ex:There are plenty of weird fish at the ocean floor.
The planks of wood in a wooden floor
ex: They just floorboards to my house.
To be realistic and practical.
ex:He doesn't love you,you must keep your feet on the ground.
Someone is sad or depressed.
ex:I didn't get my schoolarship,I am so grounded.
When a plan or business starts to succed.
ex: My company just gets on the ground