Tempera-on-wood images of Jesus washing his disciples' feet and their "last supper" together are on the back ("verso") panel of Duccio's Maestà. They are part of the set of "Stories of the Passion" included in the altarpiece of the Cathedral (Duomo) of Siena, Italy.
Jerusalem at that time - as it is today - was located near a desert. People wore sandals and walked from place to place. As a result, their feet became dusty and dirty in the dry climate. As part of the preparation for celebrating the Passover, people washed their feet as well as their hands.
Before dinner, Jesus took a basin of water and a towel and washed the feet of his disciples, despite their protests. After He had washed their feet, Jesus told his followers:
I have set you an example that you should do as I have done for you.
While they ate the Passover meal (which Leonardo da Vinci, an artist of the Renaissance, beautifully illustrated centuries later, in one of the world's most famous frescoes), Jesus told His disciples that one of them would betray Him that night.
Without identifying who it was, to the rest of the group, Jesus encouraged Judas to get on with the business at hand.