Okay kiddo, imagine you have a toy box full of different shaped toys like cubes and pyramids. Now, imagine you want to make those toys bigger or longer but keep the same shape. That's called extending a polyhedron!
Let's take a cube for example. If you want to extend it, you can add more cubes to each side of the original cube to make it longer or wider. This way, you still have the same shape but it's bigger.
The same thing can be done with pyramids and other shapes too. You can extend it by adding more of the same shape to each side or adding a new shape onto it. The important thing is to keep the same shape in order for it to still be considered a polyhedron.
So, that's what it means to extend a polyhedron - making it bigger while keeping the same original shape!