I assume they mean down to the base of the penis. It only goes down past the glans (head) of the penis.

I went ahead and made the dangerous Google search to find a good source for you. This page has a pretty clear diagram (so, uh, NSFW I guess): See https://coloradonocirc.org/foreskin/

Key quotes:

The foreskin, or prepuce, is a retractable, double-sided fold of skin and mucosal tissue that covers the glans, or head, of the penis.

The outside of the foreskin is like the skin on the shaft of the penis but the inner foreskin is mucosal membrane like the inside of the eyelid or the mouth.

The penile skin (including the foreskin) is free to move over the penile shaft because the double-sided nature of the foreskin makes the penile skin system considerably longer than the penile shaft and also because the penile skin is only connected to the underlying structures at the pelvis and at the base of the glans (at the sulcus).

This page is not as clear on this specific topic but is much more helpful for general foreskin-related health information: https://my.clevelandclinic.org/health/body/23175-foreskin#additional-common-questions