Okay kiddo, let me explain seisin to you!
Imagine that you have a toy that you really love and play with all the time. You grab it, hold it tightly, and say, "This is mine!" That's kind of like what seisin means in the world of property ownership.
Seisin used to be a way to show that someone really owned a piece of land or property. It meant that they had physical possession of it and could control what happened there. So, if you were a farmer and you had seisin of a field, that meant you could use it to grow crops or raise animals, and nobody else could come along and kick you off of it.
Today, the concept of seisin is not as important as it used to be because we have other ways to prove that we own property, like deeds and legal documents. But long ago, seisin was a way to make sure that people respected ownership rights and didn't take land that didn't belong to them.
So, to sum it up, seisin just means having physical possession and control over a piece of property to show that you own it. And, just like how you would protect your favorite toy, people used seisin to protect their land and property.