This is a medieval aquamanile or jug containing water for hand washing.
To learn more about aquamaniles:
Hand washing was an important part of table manners in medieval society. People washed before a meal and sometimes between courses. The common image of medieval nobles and knights gnawing on bones and then throwing them to the dogs would not have happened in polite society.
For a great discussion of hand washing and a recipe for a scented, herb-infused, hand-washing water:
Health professionals today still recommend hand washing.
Please consider nominating my medieval novel, The Viscountess and the Templars on Kindle Scout.