In the episode "Careers in Science," Gargantua-1 is a massive space station that has seen better days. It's described as having a geosynchronous orbit, although it isn't clear if it's also geostationary. A geosynchronous orbit means the satellite has the same orbital period as Earth. Therefore the satellite will cross the same spot in the sky (relative to an observer on Earth) at the same time every day. This doesn't mean it will appear to stay in the exact same spot in the sky, since the satellite may be orbiting at an inclination from the equator. To explain that another way: As Earth rotates, a stationary observer on the ground is moving directly from west to east, while the satellite may be moving at some north-south angle (and is not necessarily ever directly overhead). Because the orbital period is the same, the satellite "meets" the same spot in the sky at the same time each day.
A geostationary orbit is a special case in which the satellite orbits along the equator, allowing it to maintain the same relative spot in the sky. This concept is widely credited to science-fiction author Arthur C. Clarke.
In practice, a satellite (or space station) in geosynchronous or geostationary orbit needs to periodically use thrusters to keep itself in the proper orbit. SPOILER ALERT: This may explain why Gargantua-1, in serious disrepair and experiencing a problem that may or may not have been urine-related, eventually drops out of orbit and crashes.