「男の子 (おとこのこ)」 means "boy." This word refers to a male child. You use it when talking about young male humans, for example, a son, a student, or a child playing in the park.

幼(おさな)い男(だん)性(せい)の子(こ)供(ども)です。公(こう)園(えん)で男(おとこ)の子(こ)が遊(あそ)んでいます。