Phrases of prepositions often show where something is in according to someone or something else. Prepositions can also indicate time.
Prepositions often appear in phrases together with nouns or verbs. A preposition with a verb will have another meaning than the verb alone.
Prepositions are often used in time expressions, they can have another meaning than alone or in other expressions.
in - year and months, in 2013, in May
on - dates and days, on August 5th, on Monday
at - about time, at 12:15 pm, at 09:30 am
am - after midnight and before noon (ante meridiem)
pm - after noon until midnight (post meridiem)