Meaning of a vertical bar in differentiation

[kenenbek (Kenenbek Arzymatov)]

I am reading this article https://pytorch.org/tutorials/beginner/blitz/autograd_tutorial.html

Could you help what this vertical bar means?

[Abhilash_Srivastava]

This notation is used to denote the value of the partial differentiation of o with xi at the value xi = 1.
So, in the equation do/dxi = 3(xi + 2)/2, replace xi = 1, you’ll get 9/2.