This noun refers to the opening in the lower part of the human face, through which food is taken and sounds are emitted. You use your 口 to eat and speak. For example: 口を開けてください。(Please open your mouth.)

食べ(たべ)たり話し(はなし)たりする顔(かお)の開口部(かいこうぶ)です。例えば(たとえ)ば：口(くち)を開け(あけ)てください。