It's a common rough guess for 1RM. Weight x reps x 0.0333 + weight = estimated 1RM. Another way to write it would be weight x reps / 30 + weight = estimated 1RM. I first saw it when I bought Jim Wendler's ebook 5/3/1 several years ago. I'm sure he found it somewhere else first, and I imagine there are many other formulas that may or may not work for you. Feel free to reduce it to the lowest common denominator, or feel free to completely disregard and not use it at all. It works pretty well for me.