Old Church Slavonic is a very old language that was used in Europe many many years ago. It was used by people who went to church and wanted to read and write things in the language that the Bible was originally written in.
People started using Old Church Slavonic around the 9th century. It was created by two brothers called Cyril and Methodius who were from Greece. They wanted people to be able to understand the Bible in their own language, so they made up this new language that used letters from the Greek alphabet.
Old Church Slavonic was used a lot in places like modern day Russia and Ukraine. People would speak other languages like Russian or Ukrainian in their everyday life, but then they would use Old Church Slavonic when they went to church or wanted to read and write things related to the church or the Bible.
Now, people don't really use Old Church Slavonic anymore except for in very specific circumstances like religious ceremonies or academic research. It's a very interesting language though because it represents an important time in European history when people were discovering new ways to communicate and share knowledge.