An adverb clause serves as an adverb. It modifies the meaning of (or says something more about) a verb, an adjective or another adverb. Given below are two sentences with similar meanings. One of them is constructed with an adverb and the other is constructed with an adverb clause.

We must get the roof repaired now. (Here the adverb now modifies the verb get repaired.)
We must get the roof repaired before the rains set in. (Here the adverb clause ‘before the rains set in’ modifies the verb get repaired.)

Adverb clauses tell when, where, why, how, with what purpose or on what condition an action is done.

There are different kinds of adverb clauses:

Adverb clauses of time
Adverb clauses of place
Adverb clauses of purpose
Adverb clauses of condition
Adverb clauses of cause or reason
Adverb clauses of comparison

Adverb clauses of time
Adverb clauses of time tell when an action is done. They are introduced by the subordinating conjunctions when, as soon as, while, before, until, after, since, till, as long as etc.

Do not get off the train until it stops.
I will go abroad after I finish my studies.
I always switch the lights off before I step out of the room.

Adverb clauses of place
Adverb clauses of place state where an action takes place. They are usually introduced by the subordinating conjunctions where, wherever etc.

Put the keys where you can find them easily.
This is the house where my family lives.

Adverb clauses of purpose
Adverb clauses of purpose are introduced by the subordinating conjunctions that, lest, so that, in order that etc.

We eat so that we may live.
He ran fast lest he should miss the train.

Adverb clauses of cause or reason
Adverb clauses of cause or reason are introduced by the subordinating conjunctions as, since, because, that etc.

I like John because he is sincere.
As he was not feeling well, he left early.
I am sorry that I missed the concert. (= I am sorry because I missed the concert.)