From http://www.baike.com/wiki/13%E8%96%AA
13薪 is a type of salary bonus scheme. The 13 comes from "13th month". The idea is that in the last month of the year (or after working for 12 months), the last monthly salary will be doubled, as if you receive the salary for a non-existent 13th month.
Sometimes it's calculated based on total employment, so if you have only been working for half a year, the 13薪 bonus is half of what it would be.
It's a scheme that helps retention, as employees who don't work for at least 12 months may not receive this bonus.