Do you mean the diaphragm? That sits near the bottom of our ribs and forms a sheet across the whole of the inside of our body (it separates our lungs from our guts). The diaphragm is really important – without it we couldn’t breathe
Yes, I think you must mean the diaphragm. Like some other muscles in the body, it’s an involuntary muscle which we don’t control consciously (unlike for instance muscles in our arms and legs), but whose contractions occur unconsciously.