When buying the beverages for a party, this is the beer that you reserve for yourself, which is distinguished by being a mark above the rest. Can also be applied to the beer reserved for someone of importance (e.g. the individual supplying the money for the drinks or the person in whose honor your are celebrating) as a sort of gift.
Guy behind you in line: "Whoa...so much nati... this twelve pack of Becks seems a little out of place."
"Don't take that keystone, here, let me give you one of the king's beers."
"Hand's off, that's the king's beer."