Something has been done very well, or perfectly by another. Anyone trying to do the same thing after that faces a very difficult task.

In any competition, a "tough act to follow" refers to a performance so excellent, that it is not likely that a better performance can be offered.

It is an expression coming from show business. It means that the earlier performance was a very good one making the audience expect more from you in comparison. The expression can be used anytime a person who is very good at something is replaced. For example, if a respected CEO of a company retires, the person taking over the company might say that he is "a tough act to follow".