This word refers to your younger brother. It is typically used when talking about your own family. If you're talking about someone else's younger brother, you might add 'さん' (san) and say '弟さん' (otōto-san).

自分(じぶん)の弟(おとうと)を指(さ)す言葉(ことば)です。他人(たにん)の弟(おとうと)には「弟(おとうと)さん」を使(つか)います。