This seems to be a buzz word of the top leader of China and it does sound important. But what does it really mean?
大 has the meaning of 'utmost important'
国之大者 means 'the most important thing to a nation' -- the most important thing to a nation is a correct national policy and governance system (not its power)
Similarly: 俠之大者 means 'the most important quality of a swordsman' -- the most important thing to a chivalrous swordsman is his principle and conviction to justice and morality (not his skill)
“国之大者”原文为“国者、天下之大器也，重任也，不可不善为择所而后错之，错险则危”。 以上这段话出自《荀子·王霸》，用白话文解释就是，治国是极为重大的事，一定要认真选择立国之道和治国之人担当这样的重任，一旦在这方面问题上出错，国家就危险了。 可以理解为，一个国家选择正确的国策和由杰出的人才掌管这个国家，便是“国之大者”。