It is the birth date. It consists of year, month, day and hour of birth.
For example, today in 2016, June 17, 4:38 pm.
The 16th year is the year of Bingshen, June 17 is the month of Jiawu, Gengwu day, 4:00 pm 38 Jia Shen Shi
The eight characters are Bing Shen, Jia Wu, Geng Wu, Jia Shen