A preposition is a word or phrase that expresses the relationship in time or space between two things. Prepositions are commonly used between two nouns, a verb and a noun, or two verbs.
El libro en la mesa. - The book on the table.
Yo voy a Correos. - I'm going to the post office.
Ella estudia para trabajar. - She studies to work.
Daniel desayuna temprano por la mañana. - Daniel has breakfast early in the morning.
Hace dos días que no te veo.- I have not seen you for two days.
Lleva lloviendo desde ayer. - It's been raining since yesterday.
Here's a list of Spanish prepositions with audio.