Can someone tell me what it weans to challage another nation

[Panagouge]

It's a minigame you play for fun, that's all there is to it.

[Dexterra]

I'm assuming that you're referring to the (sort-of) game within the game that can be found if you visit this page or another nation's main page and click on the crosshairs between the trend and card icons, directly below the civil rights, economy and political freedoms rankings. It pits your nation against another by comparing different census rankings/site statistics; whomever scores more highly after a certain number of rounds wins. Is that what you're looking for?

^{Small word changes, all the information's the same - glad to have helped}