Okay, so imagine you're playing with your toys and you have a group of friends who speak the same language as you, but you notice that some of them say certain words differently. This is kind of like the shtokavian dialect in the Balkan region.
Shtokavian is a fancy word that describes a certain way of speaking a language called Serbo-Croatian. It's like a type of accent, but instead of just how someone sounds, it also includes some different words and changes to how the language works.
In this case, the shtokavian dialect is spoken by lots of people in countries like Croatia, Serbia, and Bosnia & Herzegovina. But even within those countries, there can be different kinds of shtokavian depending on where you live.
So just like how you might say "paw-tay-toe" but your friend says "po-tah-toe" even though you're both speaking English, people who speak shtokavian might use different words for things or say them in a different way than people who don't. But it's still all the same language, just with some different flavors depending on where you are.