You gain an extra digit on each hand and foot. The extra fingers or toes, although fully functional, do not confer any special benefit, nor do they hinder you in any way.
Modern System Reference Document © 2002, Wizards of the Coast, Inc.; Authors Bill Slavicsek, Jeff Grubb, Rich Redman, Charles Ryan, based on material by Jonathan Tweet, Monte Cook, Skip Williams, Richard Baker,Peter Adkison, Bruce R. Cordell, John Tynes, Andy Collins, and JD Wiker.