You enter an address and walkscore computes a measure of how walkable that address is from the proximity of nearby parks, schools, restaurants, etc. (based on the Google maps database).
The maximum result is 100, and they categorize the location according to the following intervals:
To a limited extent, I would think that the walkscore would be a decent measure of the bikability of the address as well, although it does not take into account important factors such as the quality of the roads for biking. But it gives you a good sense for how many destinations are within a very short bike ride.
90 - 100 = Walkers' Paradise: Most errands can be accomplished on foot and many people get by without owning a car.
70 - 90 = Very Walkable: It's possible to get by without owning a car.
50 - 70 = Some Walkable Locations: Some stores and amenities are within walking distance, but many everyday trips still require a bike, public transportation, or car.
25 - 50 = Not Walkable: Only a few destinations are within easy walking range. For most errands, driving or public transportation is a must.
0 - 25 = Driving Only: Virtually no neighborhood destinations within walking range. You can walk from your house to your car!
Interestingly my home here in Los Alamos ranks 42 out of 100, at the high end of the "not walkable" range.