3D reconstruction is a process that turns 2D pictures into 3D objects. Think of it like taking a bunch of pictures of something from different angles, and then putting those pictures together to make an object that you can look at from any angle, just like a real object. It works a bit like putting together a jigsaw puzzle. You start by using special software to figure out how the pieces fit together, and when you've found the right way, you can turn the 2D pictures into a 3D model.