How does the Pectoralis Major work when doing a bench press?
The pectoralis muscle attaches to the top to the inner half of the clavicle, just underneath the biceps on the inner part of your arm and the breastbone (a.k.a the sternum) which is found in the middle of your ribs. See below: 

So taking that in account the pectoralis muscle helps bring your arm closer to your body (adducts your arm), pulling it forward in front of you (flexes the humerus) to push up like you would when you bench press. This action can be seen below.