A runners toe is where blood pools under the toenail.
The reason that you get runners toe and have blood pool up under the toenail is because of the toe mashing up against the inside of the shoe when running.
After several miles of running and your toes getting mashed up in the front of your shoes it causes that blood in your toes to pool up.
The runners toe usually resolves on it's own without any medical treatment and usually heals within a week or two and sometimes sooner depending on how bad it is.
Sometimes a runners toe may be caused by wearing shoes that are too small but it's not always the cause.
Still make sure you are wearing proper fitting shoes with plenty of room for your toes so they don't get mashed up as bad in the shoes when running.