Let's go from easy to hard: "It's about people who love their country." "It's about people who love their country too much." "It's about people who love their country too much to be lazy." "It's about people who love their country too much to do less than they can." "It's about people who love their country too much to do less than their best." If you don't understand all of these, tell me which is the first one that you do not understand.

It they will not do 'anything less than their best', it basically just means that they will do their best.