The nautical star is a 5-pointed star, the points are split in half and filled with an alternating color (usually black-red or black-white), creating a 3d illusion.
Sailors used stars and the North Star (aka Polaris) for navigation, so it's not hard to see why so many of them have star tattoos. They function as a guide to get them back home safely. Soldiers have them for the same reason. Swallow tattoos also have a similar kind of nautical history.
Also today, people have star tattoos for similar reasons, you can rely on them when you are lost in your life, they are a light in the darkness.