Historical significance means that something is very important in history. Just like how you might learn about important people or events in school, historians look at things from the past and decide which ones are very important.
To decide if something is historically significant, historians think about a few things. They think about how much of an impact it had on the world at the time. They also think about how much it has impacted things since then. For example, learning about how people trained horses is not very historically significant because it hasn't had a big impact on the world today. But learning about the invention of the telephone is historically significant because it changed the way we communicate with each other forever.
When something is historically significant, it means we should remember it and learn from it so we can understand how our world got to be the way it is today. Historians help us do that by writing books and teaching us about important people and events from the past.