An astronaut propulsion unit is a special tool that helps astronauts move around in space. You know how on Earth, we can walk or run to get from one place to another? Well, in space, there's no gravity to help us move around. So, astronauts use a little machine strapped onto their spacesuits to help them move.
The propulsion unit has small jets of gas that shoot out in the opposite direction the astronaut wants to go. This creates a force, or push, that propels the astronaut forward in space. It's sort of like how blowing up a balloon and letting it go makes it fly around the room - the gas shooting out of the balloon in one direction creates a force that pushes the balloon in the opposite direction.
The astronaut can control the direction and speed of the gas jets using buttons and switches on the propulsion unit. This way, they can move forward, backward, up, down, and even rotate or spin in space.
The propulsion unit has to be carefully designed and tested to make sure it works properly in the harsh environment of space. It's an important tool that helps astronauts do their jobs and explore the universe - just like how your toys and tools help you play and learn!