Originally Posted by rpenmanparker

FC is "front center". RC is "rear center". They indicate how the bike is balanced and rider weight distribution since most riders will always establish a fixed saddle position relative to the bottom bracket.

Originally Posted by HTupolev

"Front center" is the distance from the bottom bracket to the front wheel axle. "Rear center" is the distance from the bottom bracket to the rear wheel axle.

Adding them together doesn't give you wheelbase unless the bottom bracket sits on a line between the two wheel axles; generally the bottom bracket is below that line, so FC+RC gives you something slightly bigger than the wheelbase.