A big grass field on an orbiting satellite is a joke which refers to a parking space.
It can be broken down into:
- A park (the big grass field)
- In space (an "orbiting satellite" is the phrase used to refer to things like planets, moons and asteroids in space, which revolve around a larger body - such as a larger planet or star).
This type of joke is known as a pun, because part of the reason is funny is that the answer to the riddle sounds like a different phrase.
Park in space sounds a lot like "parking space".
This adds to the hilarity because, as far as we know, there are no parking spaces in outer space.