If you place your hands on the last 2 ribs on each side of your body, you have found where your diaphragm is. 
The diaphragm is the main mover of air! So, when you think of breathing, tune into this amazing life giving structure. Under your hands you would feel those ribs widen and lift a bit when you breathe in, and then lower and narrow when you breathe out.
The contraction of the diaphragm expands the volume of the lungs and thereby pulls air into the body. The diaphragm is a dome shaped structure with a central tendon and muscles that attach to the inner walls of the ribcage and the spine.
a simplified concept would be to think of a trampoline with it’s bouncy middle (central tendon) and springs attached all the way around it (the muscular attachments). The springs are the muscles contracting (inhale) or relaxing (exhale). In the feature picture the central tendon is white and the muscles are red.