Old French is like the language your great-great-grandparents used to speak. A long, long time ago, people in a country called France used to talk in ways that might seem strange to us today. They had different words and different ways of saying things. Some people who study languages have given this way of talking a name - "Old French."
So imagine you have traveled back in time hundreds and hundreds of years to France. You hear people talking, but it doesn't sound like the French language you might know. It's because the words are different - like instead of saying "Bonjour" to say hello, they might say "Salut." They might also use different grammar, which means they arrange words in sentences differently.
People don't speak Old French anymore, but we can still study it to learn more about the language and how it has changed over time. We can read books and stories written in Old French and see how it's different from the French we use today. It's like a treasure that helps us understand how people used to communicate long ago.