This word refers to your younger sister. It is typically used when talking about your own family. When referring to someone else's younger sister, you usually add 'さん' (san) and say '妹さん' (imōto-san).

自分(じぶん)の妹(いもうと)を指(さ)す言葉(ことば)です。他人(たにん)の妹(いもうと)には「妹(いもうと)さん」を使(つか)います。