It is not clear whether this is something they will learn later, after His resurrection, or whether this refers to what Jesus tells them a few verses later in this chapter:

12 So when He had washed their feet, and taken His garments and reclined at the table again, He said to them, “Do you know what I have done to you? 13 You call Me Teacher and Lord; and you are right, for so I am. 14 If I then, the Lord and the Teacher, washed your feet, you also ought to wash one another’s feet. 15 For I gave you an example that you also should do as I did to you. 16 Truly, truly, I say to you, a slave is not greater than his master, nor is one who is sent greater than the one who sent him. 17 If you know these things, you are blessed if you do them.

Shalt know hereafter - Jesus at that time partially explained it John 13:14-15; but he was teaching them by this expressive act a lesson which they would continue to learn all their lives.

Every day they would see more and more the necessity of humility and of kindness to each other, and would see that they were the servants of Christ and of the church, and ought not to aspire to honors and offices, but to be willing to perform the humblest service to benefit the world.

And we may remark here that God often does things which we do not fully understand now, but which we may hereafter. He often afflicts us; he disappoints us; he frustrates our plans. Why it is we do not know now, but we yet shall learn that it was for our good, and designed to teach us some important lesson of humility and piety. So he will, in heaven, scatter all doubts, remove all difficulties, and show us the reason of the whole of his mysterious dealings in his leading us in the way to our future rest.

We ought also, in view of this, to submit ourselves to him; to hush every murmur, and to believe that he does all things well. It is one evidence of piety when we are willing to receive affliction at the hand of God, the reason of which we cannot see, content with the belief that we may see it hereafter; or, even if we never do, still having so much confidence in God as to believe that what He does is right.