A preposition usually connects two things: a noun, adjective or verb that comes before it and a noun phrase or pronoun that comes after it.

He looked at her. (Here the preposition at connects the verb looked and the pronoun her.)
He put the books on the table. (Here the preposition on connects the nouns books and table.)
I don’t know why she is angry with me. (Here the preposition with connects the adjective angry and the pronoun me.)

In some cases the preposition may come at the end of the clause.

Wh-questions

When a question word is the object of a preposition, the preposition usually comes at the end of the clause.

Who are talking to? (More natural than ‘To whom are you talking?’)
Who’s this gift for? (For whom is this gift? is extremely formal.)
Where did you buy this from?

Relative clauses

When a relative pronoun is the object of a preposition, the preposition usually goes at the end of the clause. This is common in an informal style.

This is the boy that I told about. (Less formal than ‘This is the boy about whom I told you.’)
That is what I am worried about.
He is the only man who I have ever been in love with.

Notes

Some prepositions cannot come at the end of clauses. Examples are: during and since.

Since when have you been working here? (NOT When have you been working here since?)
During which year did it happen? (NOT Which year did it happen during?)