Like he said, 3d mark is a benchmarking program to help you know how better your computer stacks up against the best computer configuration out there. How it does it is by trying to render progressively higher resolution/higher polygon count/higher shader quality scenes and then observing the frames per second your computer is able to produce. Different calculations (physics,sound,shader,etc) are carried out by different parts of your computer.
By applying some proprietary weighted algorithm on the results, the program is able to compute out a number called your computer's score, which can be used to quantitatively compare your computer against the other computers in the wild.