Accuracy means how well Presto was able to able to locate the target, based on the address information that you gave. Even if you supply only partial information, Presto's geolocation service can usually make a very good guess at where your target is!
The accuracy ratings are:
3 stars: Presto was able to locate the exact address.
2 stars: Presto was able to locate the target to within the street or zip code.
1 star: Presto was able to locate the target to within the city.
A warning sign indicates that Presto was unable to find your target's city. This is considered a "possibly problematic" Target.
Do you need three stars? Not necessarily -- sometimes our geolocation service is confused by something that a human being has no trouble figuring out. An evaluator can usually figure out where "Happy Burger, Smalltown" is, as long as Smalltown really is a small town. If it's "Happy Burger, Los Angeles", the evaluator is probably going to need at least a street to find the location.