Read John 13:1-17
His act was shocking to say the least. As Jesus rose and took off his outer cloak and took the towel and basin his followers glared in disbelief. One-by-one he went from disciple to disciple and did the one job that only the lowest and most base servants were to do. He washed their feet.
This might not trouble you at all. Washing feet in our culture is simply a pedicure. Feet aren't that gross. But in the first century the feet were where the funk was. Bacteria, raw sewage, infestation, mites, disease, to say nothing of dust were all the things that hung out on the foot of a first-century Israelite. Furthermore, the position of reclining to eat at a meal meant that you had someone else's feet pretty close to the proximity of your food and your face. Because of this the washing-feet job was the job for the most menial servant, not the primary leader.
That's what made Jesus' action so shocking. At least 12 other guys down the ladder were supposed be in line for the job. Yet, the greatest was serving the least.
Peter's objection is somewhat commendable. "Lord, do you wash my feet?" He gets it in theory. But notice he didn't volunteer for the job. He knew Jesus shouldn't be doing this job, yet he wasn't humble enough to do it himself.
Jesus' explanation of his actions shows us the heart of the gospel. He came to serve. He being God-in-flesh, humbled himself, took the form of a servant and served. Mark's Gospel reminds us that the "Son of Man came, not to be served, but to serve and give his life as a ransom for many" (Mark 10:45).
The King over all kings served. The point of Maundy Thursday is for us to reflect on his service. It's to see through Jesus' work of foot washing the larger picture of his service. It's not just an example for us that we should care for and serve one another (although Jesus does instruct us to that). This is a small glimpse of the greater service that he is undertaking for all his children. It is a minuscule act that points forward the major act.
The King washes feet. The King goes to die. The King serves.