Old Bulgarian is a way of writing and speaking in Bulgaria that people used a long time ago, kind of like how adults used to talk differently when they were younger. It's like an old secret code that not many people know how to use anymore.
Back in the day, instead of using a normal alphabet like we do today, they used a different way of writing called the Cyrillic alphabet. It has some letters that look different than the ones we usually use.
Old Bulgarian was also the way people used to speak in Bulgaria a long time ago. They didn't use the same words or grammar as we do now. It's kind of like listening to your grandparents tell stories in a different language.
Today, Old Bulgarian is mostly used for studying and historical purposes. It's not really used for everyday writing or speaking anymore because it's outdated and difficult for people to understand.