Okay kiddo, so hobby injection moulding is when someone who likes to make things as a hobby, uses a special machine to make things out of plastic.
First, they take a special kind of plastic and melt it down to make it really hot and gooey. Then they put it into a special machine called an injection moulding machine.
Next, they use a special mould that has a shape of something they want to make, like a toy or a little figurine. They put the mould into the machine and the hot plastic gets injected into the mould.
Once the plastic cools down and hardens, they take the mould out of the machine and they have a cool new thing that they made all by themselves!
It's like making your own toys or souvenirs, but instead of using clay or paint, you use melted plastic and a special machine. Pretty cool, huh?