Complex sentence: Because of this importance, Holland served as a "pars pro toto" for the entire country, and is considered either incorrect or informal, depending on the context, but is more acceptable when referring to the national football team.
Split into simpler sentences: Because of this importance, Holland served as a metonym for the entire country. Referring to the Netherlands as Holland is an example of "pars pro toto" and is considered either incorrect or informal, depending on the context, but is more acceptable when referring to the national football team.