"Supinated" refers to the position of the hand or foot when it is turned or rotated outward, with the palm of the hand facing up or the sole of the foot facing outward. Full definition
Using a close grip with supinated hands is easier than using a wider grip in pronation; however, the biceps work more and the back works less with this version. (bodybuilding-wizard.com)
This happens because the narrow grip forces the biceps into a more supinated position than wider grips (supination is the movement where the hands rotate to a palms - up / forward position). (muscle-fitness-training.com)
Lusk et al. (2010) compared the lat pull - down performed with a pronated and supinated hand - grip on latissimus dorsi muscle activity. (strengthandconditioningresearch.com)