The disciples were sitting on the floor for the meal, when Jesus stands up and takes the towel to wash their feet.
Every ones feet needed washing because their feet were dirty from walking on the roads littered with dung, left behind by the passing flocks of sheep and other animals.
At meals people would recline on the floor around the food, their feet not far from other guest’s noses. Imagine trying to fellowship with Jesus but all you can think of, is your bad smelling feet and the feet of your closest table guest. You hope nobody notices but you are distracted and embarrassed.
The disciple’s feet were presumably already washed before the meal, so Jesus is making a point and it is plain; I have washed your feet, so you can and must, forget the failures and the shame of your past walk. Your fellowship with me is based upon what I have done for you, not how you have walked. You may be disappointed about your past but leave it. While you are conscious of the past and where you have been, you’re not able to hear what I’m saying to you, nor enjoy my fellowship with you.
Friend, our acceptance is not based upon our performance but His perfect cleansing work and our fellowship is entirely the result of what He has done, not what we have done.