"Defense" involves positional adjustment to provide total defensive value. "UZR" is a measure of the player's actual fielding, no positions involved.
So if you want to see how good Donaldson is at defense, you'd look at his UZR, and it's +10 runs or whatever. So that means his glove has been worth 10 runs or like 1 win. But if you want to see his total defensive value, you'd look at defense, which includes the fact that he's a +10 defender at third base; obviously, if he was a +10 defender at first base, that wouldn't be as impressive and would impact his total defensive value, despite posting the same UZR. Hope I explained well