Remember that defense is hard to measure, but by accounting how many balls that are hit to a fielder he can convert into outs is the most accurate way to define how good his defense is.
Since i'm lazy, i'll give you a link that explains the UZR formula better than i could.
http://www.baseballthinkfactory.org/files/primate_studies/discussion/lichtman_2003-03-14_0/