Okay, so imagine a playground where all the kids have different toys. Some kids have really cool and expensive toys while others only have simple ones. Third-worldism is a way of thinking that says the kids who have simple toys should work together because they are all in the same situation. They might not have as much as the kids with cool toys, but they can still play together and have fun.
In the real world, third-worldism is a way of thinking that believes that poorer nations should work together to improve their situations. They might not have as much money or resources as richer nations, but by working together they can try to make things better for their citizens. It's like the kids with simple toys banding together to make their playtime more fun and enjoyable.