What do prepositions do?

Prepositions describe the relationships between words.

 Preposition of place answer questions of where.

The moon is in the sky. Where is the moon? It is in the sky.

Preposition of time answer questions of when and how long.

They played chess after playing checkers.   When did they play chess? After playing checkers.

I slept over five hours. How long did you sleep? Over five hours.

 

source: http://www.k5learning.com/blog/playing-prepositions-place