Not just countries, but cities, people, and really anything you've never personally experienced before. It's easy to put labels on things to make them easier to understand, especially when you've never come face to face with them. Too often this leads to a limiting point of view that denies the opportunity for growth and fosters ignorance. So travel, to another country, the town an hour away that you've never been to, or to a social gathering with people you've never met.
The more you learn about anything, the more you understand everything.