Jesus rises from supper, lays aside his outer garments, wraps himself with a towel, pours water into a basin, and begins washing and drying the disciples' feet. John narrates the physical procedure in unusual detail.
The shock lies less in the existence of foot washing than in who performs it.
Dusty roads and open footwear made washing feet a practical courtesy in the ancient Mediterranean world. Hosts could provide water, while the actual washing could be performed by servants or persons of lower household status. Removing an outer garment, girding with a towel, and using a basin are practical actions that also visually place Jesus in the role of a servant. Foot washing was not uniquely Jewish, but it appears naturally within biblical and Jewish hospitality traditions.
The cultural background explains Peter's astonishment. A disciple might reasonably serve a master; here the recognized Lord and Teacher kneels to wash his disciples.
Jesus turns an ordinary low-status task into a enacted definition of leadership. His authority is not diminished by service; it is displayed through service.