Okay kid, so the world is made up of many countries and some countries have more money and resources than others. The countries that are really rich like the USA and Europe are called "first world" countries. The countries that are kind of rich like China and India are called "second world" countries. And the countries that are not so rich, and maybe have a lot of problems like poverty, war, or lack of infrastructure, are called "third world" countries.
Third world countries are still developing and catching up to the rest of the world. They might not have a lot of technology, education, healthcare, or infrastructure like good roads and clean water. People living in third world countries might struggle to find food and shelter, and their governments might not be strong enough to help them.
It's important to remember though, that just because a country is considered third world, it doesn't mean it's not a great place with wonderful people and a unique culture. We should treat people from all countries with respect and kindness, no matter where they come from.