There are two meanings for the word "tough": difficult; and durable. When the going gets tough = when your situation/progress becomes difficult The tough get going = the people who are strong and durable start to work. Bonus music: www.youtube.com/watch?v=fg3vzl_VwLc

There's really nothing to explain here. The meaning is obvious. the going = 情况 get going = start doing things Let's get going = 去吧。

the meaning of the phrase is "When the situation becomes difficult, the strong will work harder to meet the challenge." :)