An adverb clause is a subordinate clause that modifies a verb, adjective, or adverb. It answers one of four questions: how, when, where, and why.
An adverb clause usually begins with a subordinating conjunction.
Subordinating conjunction: He sang because he was happy. (Clause beginning with because modifies verb sang.)
Relative pronoun (rare): He was sad that his friend had to move. (Clause beginning with that modifies adjective sad.)