「女の子 (おんなのこ)」 means "girl." This word refers to a female child. You use it when talking about young female humans, such as a daughter, a student, or a child playing in the park.

幼(おさな)い女(じょ)性(せい)の子(こ)供(ども)です。私(わたし)の妹(いもうと)は女(おんな)の子(こ)です。