Pick a star, any star and watch it for a while. It moves. As the Earth spins all the stars move across the sky, taking 23hr 56 min to move all around the sky to the same place. This is due to the rotation of the Earth (less than 24 hours due to the orbit of the Earth). We sometimes say the "celestial sphere" appears to rotate around the Earth, due to the Earth's rotation.
The planets also move across the sky every day, taking just less than 24 hours to all around the sky.
But the planets also move relative to the celestial sphere, and since th stars don't move relative to the celestial sphere we can say that the planets move relative to the stars. It is this motion that is of interest, since it is not due to the rotation of the Earth, but to the actual motion of the planet relative to the Earth. This motion is much slower than the apparent motion of the celestial sphere; Jupiter takes 12 years to make a full journey around the sky. But this motion is special, complex and interesting. It is this motion that earns planets the name "wandering stars" in ancient Greek.
The stars are at a very great distance, so to all intents and purposes they are "fixed". Relative to each other, the stars hardly move at all. You would need to wait centuries before you noticed any difference. So the background of fixed stars makes a very convenient way to refer to the motion of the planets