Prepositions are those small very important function words that come before nouns to form phrases and need to be used correctly, so that the phrase makes sense.  For example: in, on, at, by. 

Location / Place
There are 3 people in the room (in = an enclosed space)
My keys are on the table (on = a surface)
I’ll meet you at the station, (at = a place)
I put my bag by the door (by = next to/ close to)

Time
I’m starting a new job in September (in = sometime in that month / a contained space of time)
Let’s meet on Monday (on = the specific day of the week)
The movie starts at 6pm (at = a specific time)
Can you please finish the report by lunch time? (by = at the latest / it’s a deadline)