Surprisingly enough, a foreskin piercing is a body piercing that goes through the foreskin of the penis. (Despite the name, the term is commonly also used to name a piercing through the skin of the penile shaft.) It is a surface piercing, but because the skin of the penis does not have any fatty layers (and perhaps also due to the shape of penises), the piercing is more prone to rejection than other surface piercings.
(It is worth noting that piercings along the underside of the penis and positioned parallel to it are called frenums.)
It's not just for you uncut men, either — circumcision does not completely remove the foreskin, so a trimmed man can still get one of these in the traditional locale. (Of course, if we're talking about penile shaft skin piercings, they're available to any men.)
As with any piercing, the reasons for getting one vary with each person who gets one. It is a cosmetic piercing, but can be be quite pleasurable when gently played with (once it has fully healed). Depending on the position and size of the piercing, it may also offer stimulation for a sexual partner (though a piercing in some positions may be rather painful for the man).
So, does it hurt? As far as male genital piercings go, this is one (if not the) least painful. (Though, along with pain thresholds, sensitivity of different areas can vary considerably from man to man, so one man's "That wasn't so bad..." may be another man's "Jesus H. Christ, what was I thinking??") After getting the piercing, sexual activities should be gently eased back into after three weeks; full healing time is around six weeks with proper care. Keep dirty stuff (underwear, water, sexual partners) away from it.
(Yet another piercing node brought to you by my possibly-unsettling knowledge of strange piercings!)