Let's imagine you're playing with your blocks at the park. You've got a red block and a blue block, and you're pretending that they're houses. You put the red block "on" the grass, and the blue block "next to" the sandbox.
When we talk about where things are, we use different words depending on their location. For example, "on" means the object is resting directly on top of something else, like the red block on the grass. "Next to" means the object is very close to, but not touching, something else, like the blue block next to the sandbox.
In some languages, like Polish, Russian, and some Native American languages, they have a special way of showing these relationships between objects. They have a special form of the word that changes depending on where the object is located. This is called the "locative case".
For example, in Polish, the word for "house" is "dom". But if you want to say "in the house", you would say "w domu", with the "u" at the end showing that the house is the location of the action. Similarly, if you wanted to say "on the table", you would say "na stole". The "-e" at the end of "stol" changes to "-e" to show that it's the location of the action.
So basically, the locative case is a special form of the word that tells us where an object is located or where an action is taking place, and it changes depending on that location.