a whole new ball game 字面意思是「一场全新的棒球比赛」。引申为全新的局面或者截然不同的东西。

【例句】
I go jogging every weekend, but running a marathon isa whole new game.
我每个周末都去跑步，但跑马拉松的感觉就完全不一样了。

Some people may be good at speaking English in terms of day to day conversation, but working as a professional translator isa whole new ball game.
有些人日常交流英语说的还不错，但做一名专业的翻译，那就是截然不同的事情了。