Okay kiddo, so you know how when we go to church, the pastor usually has a sermon prepared to talk about? Well, sometimes they might not have anything written down, so they have to do something called extemporaneous preaching.
This means they have to make up what they're going to say as they go along! It can be kind of like when you're telling a story to your friends, and you don't know what's going to happen next but you just keep going and make it up as you talk.
Some pastors are really good at extemporaneous preaching because they know a lot about the Bible and religion, so they can come up with good things to say on the spot. But it can be hard too, because they have to speak for a long time without making any mistakes or repeating themselves too much.
Overall, it just means that instead of reading from a script or plan, the pastor is speaking from their heart and sharing what they think is important for people to hear.