Okay, so imagine you have a bunch of toys in a toy box, and you want to play with all of them at the same time, but it's easier if you take them out of the box and put them on the ground where you can see them better. Importing a scene in a game is kind of like taking the toys out of the toy box, but instead of toys, it's things like characters, backgrounds, and special effects that you want to use in your game.
So, let's say you're making a game where a character needs to run through a forest. You can import a scene that has trees, bushes, and other forest elements already made and ready to use. This saves you a lot of time because you don't have to make those things yourself from scratch.
Once you import the scene, you can move around the elements like trees and bushes to create the forest however you like. You can also add your character to the scene and make them run through it.
Importing a scene is like borrowing someone else's toy to use in your game, but you still get to play with it in your own way.