In the context of language, pars pro toto means that something is named after a part or subset of it or after a limited characteristic, which in itself is not necessarily representative of the whole.
Pars pro toto usage is especially common in political geography, with examples including "Russia" or "Russians", referring to the political institution (both historially and contemporary) or its people; "Holland" for the Netherlands; and, particularly in languages other than English, using the translation of "England" in that language to refer to Great Britain or the United Kingdom.
Among English-speakers, "Britain" is a common pars pro toto shorthand for the United Kingdom.
An example of a pars pro toto in geography is the use of the capital to refer generally to the country such as Washington for the United States, Beijing for China, or Moscow for Russia.
When the capital is used to refer specifically to the country's government, the figure of speech is a metonymy rather than a pars pro toto.