Hello there! So, a model steam engine is like a little machine that runs on steam, kind of like a toy train, but unlike a toy train, it doesn't have wheels, it just sits still while it works.
Steam is like water that has been heated up a lot so that it turns into a special kind of gas. The steam comes out of a small hole in the model steam engine and pushes against a little piston (a round piece of metal that moves back and forth) inside.
When the steam pushes the piston, it makes a little crankshaft (like a handle that turns round and round) move too, and this makes other parts of the steam engine move. And then, if you put a small amount of oil on the moving parts of the engine, it doesn't make any noise and runs more smoothly.
So in a nutshell, a model steam engine uses steam power to make things inside of it move and turn, allowing it to do things like spin gears or even sometimes power pumps or generators. It's kind of like a little magical machine that relies on steam to get things done!