Lanula

The white-colored, half-moon-shaped structure at the base of our fingernail is called the ‘Lunula.’ This forms part of the nail matrix, basically the region that’s responsible for the growth of the entire fingernail. The rest of the nail matrix lives under your skin at the base of your fingernail.

Damaging the nail matrix will screw with the growth of the entire nail. You'll sometimes see lines called Beau's Lines if you damage the lunula.

By looking at the distance of these lines from the nail matrix, you can tell approximately how long ago the injury happened. Fingernails grow ~1mm per month and are fully replaced every 6 months. So an injury that has grown halfway up the nail (~3 mm) is about 3 months old.