The foreskin is the loose part of the skin covering the tip of the penis. It is the part that is cut during circumcision. However, the loose bit of skin actually attaches a bit behind the tip, not at the base of the penis. Think of a circumcised penis. That ring of skin just shy of the tip is where the foreskin used to attach.

Attempting to retract the foreskin to the base of the penis is painful, not entirely possible, and with enough force with tear.

This is a joke implying that someone tried to retract it to the base.