The words or groups of words that show the relation of nouns or pronouns (by being positioned just before them) with other words in the sentence are called Prepositions.
In simpler words, a preposition relates a noun or a pronoun with other words in a sentence.
- The crow sat on the branch of a tree.
- There is another Red Fort situated in Agra.
- There is a toy-shop near my house.
- Some of the students did not look at the blackboard.
- A dog ran after the cat.
In the above sentences, the highlighted words (on, in, near, at, and after) show some relationship between the nouns or pronouns that they precede and the other words of the respective sentences. Therefore, we can say that all of these highlighted words are Prepositions.