A space dock is like a giant space garage for spaceships. Just like how cars need a garage to park in and get fixed, spaceships need a space dock to park in and get fixed or upgraded.
Imagine a really big floating platform in space with arms sticking out that can grab onto spaceships and hold them in place. The space dock can then use special tools and machines to repair or upgrade the spaceship, just like how mechanics fix cars in a garage.
Sometimes, spaceships need a lot of work done, and they can't just park in any old space dock. They need a bigger dock that can handle larger spaceships with more complicated repairs.
Overall, a space dock is a very important place for spaceships to get the care they need to keep exploring the vast universe.