Cyrillic script is a way of writing letters that some people use in their language. It's kind of like using a secret code or a special way of spelling words that only people who know how to read it can understand what it says.
Instead of using the letters we're used to like A, B, C, etc., Cyrillic script uses different letters that look different. For example, the letter that looks like our letter "A" in Cyrillic script looks like a triangle with a line coming down from the middle.
There are many languages that use Cyrillic script, like Russian, Ukrainian, and Bulgarian. It was created a long time ago by two brothers named Cyril and Methodius, who wanted to make it easier for people to read and write in their own language.
So if you ever see someone writing in Cyrillic script, it might look like they're writing in a secret code, but really they're just using a different way of writing that's special to them!