Words like in, out, up, down etc., are not always prepositions. Read the sentences given below.
He was driving down the street.
Please sit down.
He climbed up the stairs.
She is not up yet.
He is in the room.
You can come in.
In the expressions ‘down the street’, ‘up the stairs’ and ‘in the room’, the words down, up and in are prepositions. Note that prepositions are always followed by nouns which act as their objects. For example, in the sequence ‘down the street’, the noun street is the object of the preposition down.
In the expressions ‘sit down’, ‘she is not up’ and ‘come in’, the words down, up and in have no objects. They are adverbs and not prepositions.
Small adverbs like these are often called adverb particles or adverbial particles. Examples are: above, about, in, out, up, down, before, across, off, on, below, behind etc.
Note that many words of this kind can be used as both adverb particles and prepositions.
Adverb particles are sometimes used together with verbs to form two-word verbs. These are often called phrasal verbs. Examples are: break down, put off, work out, give up etc.
Could you please switch on the lights?
Note that the meaning of a phrasal verb is not always guessable from the meanings of the individual words in it.