An adverb which introduces a relative clause is called a relative adverb.  Examples are: when, where, why, whatever, wherever etc. Study the following sentences.

I know the house where he lives.
There must be some reason why he cried.
Can you tell me how it is done?
The day when I met Jane was the best day of my life.

Here the words where, when, why and how are relative adverbs because they introduce the relative clauses that follow immediately.

Relative adverbs act as subjects or objects inside relative clauses, and at the same time they connect relative clauses to nouns or pronouns in other clauses – rather like conjunctions.

Notes

The relative adverb when can be replaced by ‘in/on which’. Where can be replaced by ‘in/at which’. Why can be replaced by ‘for which’.

I don’t know the day when he came. (= I don’t know the day on which he came.)
I know the house where he lives. (= I know the house in which he lives.)
Do you know a shop where I can buy used laptops? (= Do you know a shop at which I can buy used laptops?)
Do you know the reason why she cried? (= Do you know the reason for which she cried.)