When something goes on and on and on and on and on, the same way, for a long time, that's monotonous. Monotonous things are boring and repetitive, like that long story you've heard your brother tell a hundred times before.
Ever hear a song that just sounds like the same thing over and over? Ever hear a dog that keeps barking all day? Or a voice that's dull and unchanging in tone? All those things are monotonous: they don't change much, and they're dull as dishwater. Anything tedious or humdrum is probably monotonous. If someone says you're being monotonous, try to vary your tone of voice or the things you're talking about.