Okay kiddo, so an encyclopedia is like a big book that has lots of information about different things.
The Encyclopedia of Yugoslavia is a book that talks about a country that used to exist in Europe. Yugoslavia was made up of different places like Serbia, Montenegro, Croatia, Slovenia, Bosnia and Herzegovina, and Macedonia.
The book has information about what Yugoslavia was like, its history, the people who lived there, and lots of other things. So it's kind of like a big window into a place that doesn't exist anymore.