The "Resilient Population" card reads:
Take a card from the Infection Discard Pile and remove it from the game.
When you play the "Resilient Population" card, what exactly are you allowed to do?
The "Resilient Population" card lets you remove one city from the discard pile. If you are afraid you're about to get an Epidemic (or you have just gotten one!) and have the existing discard pile shuffled back onto the top, you can remove the most dangerous city so that there's no risk of an outbreak.