Well don't we just learn new things about the human penis every day?
Because the foreskin is a double-layered fold of skin, the two layers, although continuous with one another, have their own separate names. The underneath layer is called the inner foreskin and the skin on the top, the outer foreskin.
The foreskin is specialised tissue and functions to cover and protect the glans (head of the penis) and meatus (opening of urethra) of a flaccid dick. And on a hard dick the foreskin can act as a natural lubricant, and potentially a source of pleasure, due to the presence of nerve receptors.
While the outer foreskin is a continuation of the shaft skin, the inner foreskin is more like the inside of an eyelid as it's a mucous membrane.
The inner foreskin is only visible when the foreskin is retracted (pulled back), which occurs naturally with a boner.
The inner foreskin can be a different shade from the rest of the penile skin, especially evident after puberty. That's why circumcised guys often develop a kind of ombré penis, because the skin at the top of the shaft is actually the inner foreskin.