A more thorough explanation:

The shorter-term rule is a provision of the Universal Copyright Convention that states that no member country is required to extend a longer term of protection than the work receives in the country where it is first published.

For example, if a book is first published in the United States and is protected by copyright for 70 years after the author's death, but in another country, the same book is only protected for 50 years after the author's death, the shorter-term rule would apply. This means that the book would only be protected for 50 years after the author's death in the other country, even if it is still protected for 70 years in the United States.

The shorter-term rule is important because it helps to ensure that copyright protection is consistent across different countries. It also helps to prevent copyright owners from taking advantage of longer copyright terms in one country to prevent others from using their works in other countries.