Overclocking RAM refers to increasing its clock frequency beyond the factory default. For instance, setting the frequency from 1600 MHz (DDR3-1600) up to 1866 MHz or higher.
Yes, basically, it is speed. Whether it actually makes the RAM noticeably "faster," though, is a lot less clear sometimes.
Side note: Is this really a question for the Graphics & Display forum? :P
I just typed the question on the side and it automatically put it in the graphics and displays forum, does it really bother you that much to comment