the whole nation; all the people in the land
A formal, somewhat literary term referring to every person in a country or realm. Often used in historical or rhetorical contexts.
I pray for the peace and well-being of all the people in the land.
That policy brought benefits to the whole nation.
国民 is the standard modern term for 'the people of a nation' or 'citizens', while 天下万民 is more formal, literary, and often carries a historical or rhetorical tone.
万民 alone also means 'all the people', but 天下万民 emphasizes the entire realm or nation more explicitly.
Compound of 天下 (てんか, 'the whole country, the realm') and 万民 (ばんみん, 'all the people'). The exact historical derivation is uncertain, but it is a classical Chinese-style expression.