the whole nation; all the people of the country
Refers to the entire population of a country collectively, often used in political or statistical contexts.
The whole nation paid attention to the news.
This policy affects all the people of the country.
全国 means 'the whole country' as a geographical or administrative area, whereas 全国民 specifically refers to the people living there.
Compound of 全 (ぜん, 'all') + 国民 (こくみん, 'people of the nation').