Old Frankish is a language that people used to speak a very long time ago. It's kind of like when you were a baby, and you learned how to talk by copying the sounds you heard around you. People who lived in a place called Frankia, which is in Europe, did the same thing.
Old Frankish was spoken from around the 4th century AD to the 9th century AD, so it's a really old language that people don't use anymore. It's considered a Germanic language, which means it's related to other languages like English and German that people still speak today.
Some old Frankish words are similar to words we use in English now, but they sound different. For example, the old Frankish word for "father" was "fadar," which sounds like "fa-dar."
People don't really speak or write in Old Frankish anymore, because it's been replaced by other languages. But some people who study history like to learn about it, so we can understand how people used to talk and communicate with each other a long time ago.