The small dimples formed on the lower back are called Dimples of Venus and they are created because of the stretching between spine and skin.
These dimples are a sign of good blood circulation and a healthy body. Thus they are considered as a beautiful sign. Apparently, people who have dimples find it easy to reach an orgasm because of the rapid blood circulation. Since they are located at the area with no muscles, it is impossible to get these with exercise or any sort of hard work but one can easily make them visible if they cut down some fats.