Okay kiddo, so Czechoslovakia and Yugoslavia were countries that were next to each other in Europe. They used to be friends and trade with each other, but then things got a little complicated.
You know how sometimes you have a fight with your best friend and you stop talking to each other for a while? Well, that's kind of what happened between Czechoslovakia and Yugoslavia.
There were some political disagreements about how their countries should be run and about which countries they should be friends with. This disagreement led to a long period of not talking to each other very much.
Eventually, the leaders of both countries decided to try to work things out and become friends again. They started talking to each other again and even helped each other out in some ways. But even though they were friends again, they weren't exactly as close as they used to be.
So, that's the story of Czechoslovakia and Yugoslavia's relationship. Sometimes friends have disagreements, but it's always good to try to work things out and be friends again if you can.