With a nipple piercing they make a small hole in the nipple through which you can wear a piece of jewellery. In ancient Rome men in military service had a nipple piercing as sign of bravery, in the Victorian era women wore it to give the illusion of big nipples and nowadays people have nipple piercings because they like the looks of it or the feel of it or they do it as as part of a SM game.
The piercing can be placed vertically or horizontally or diagonally. Most are place horizontally. Some people have more than one nipple piercing.
If you ever have a nipple pierced inquire about safety and hygiene practises and talk beforehand about how far back you want it.
The wound normally heals within 6-8 weeks. But you have to treat it well to prevent infection. Ask for detailed written instructions beforehand. Metal allergies can cause piercing infection or rejection.
Never have it done right before a vacation in which you plan to swim, for then you risk infection.
At first the nipples are very sensitive. They can remain more sensitive than before. For some this is an advantage, others don’t like it.

See also: genital piercings