"Trap muscles" refers to the trapezius muscles, which are a pair of muscles located on the back of your neck and upper shoulders. These muscles help with the movement and stability of your neck, head, and shoulders. Full definition
If you want to build big trap muscles, use super heavy weights, and if you just want to support your posture and shoulder joint use lighter weights. (all-about-arm-exercises.com)
Get under the bar and rest it upon your upper traps muscles. (kingofthegym.com)
Also, because of the difference in arm position, the lower - lat and lower - trap muscles work harder in the chin - up, while the rear (posterior) delts, middle traps, and upper lats work harder in the pull - up. (bodybuilding-wizard.com)