Subordinate clauses are dependent upon independent clauses or other superordinate clauses. They cannot stand on their own. Often they begin with words (subordinating conjunctions, relative pronouns or question words) that connect them to the superordinate clause to which they refer.

"während" is a conjunction that introduces a temporal clause. Temporal subordinate clauses provide a time context to the action conveyed in the independent clause. They answer the question "Wann?" (when?). The conjunction "während" indicates that the occurrences in the independent and dependent clauses happen at the same time. The subordinate clause describes the span of time in which the independent-clause action takes place.

Example:
Julia hat Traponisch gelernt. — Wann?

Julia hat Traponisch gelernt, während ich gearbeitet habe.
(= Harry hat gearbeitet, und in dieser Zeit hat Julia Traponisch gelernt.)