A scrub is a now generalized term used as a synonym for a "noob" or "newb," which is someone who is bad at a video game or activity in general.
Most of the definitions here, surprisingly, have nothing to do with the actual term. The original definition is related to a person who makes a mistake in a video game, which is such a bad mistake that it is clearly wrong, yet they persist in making it. The term derives from Street Fighter II, to describe some players that were so bad that they would mash their hands across the control pad, an act known as "scrubbing," because it relates to scrubbing a car or other object with a sponge. Thus they were deemed "scrubbers," or "scrubs" for short. Over time this term expanded throughout the gaming world, and then the real world, and lost its original meaning.
If you're a scrub, then you owe me a cookie.