What do geostationary satellites do?

Geostationary satellites are objects (usually machines) that orbit the Earth at exactly the same velocity as the rotation of the Earth (strictly, the orbital period of the satellite is equal to the rotational period of the Earth). This means they stay above the same point on Earth at all times throughout it's orbit. These are used for things like communication, monitoring the weather, and navigation as they're visible to a large area of the Earth.