開く (あく) is an intransitive verb meaning 'to open.' It describes something opening on its own, without a direct agent. For instance, 'ドアが開きます' (Doa ga akimasu) means 'The door opens.' It focuses on the state of something becoming open.

開(ひら)くは「開(ひら)く」という自動詞(じどうし)です。何(なに)かが自然(しぜん)に開(ひら)くときに使(つか)います。「ドアが開(ひら)きます」