When ancient people observed the sky, they would see the planets, these 'wandering stars', which seemed to move against the backdrop of the fixed stars. The fixed stars are the stars that are so far out in space that they appear not to move at all, and some of them hold great significance when they are in alignment with planets or angles in your chart.
Of course, there are too many stars to catalogue and keep track of in a chart, and if we considered all of them in our chart, it would be so dense we wouldn’t be able to tell them all apart. While it can be argued that every astronomical object has meaning, prioritizing the most significant is very helpful. The most significant fixed stars that are considered in astrology are the ones that have always appeared to be rather bright, which makes sense since these were the ones that have caught our attention in observing the sky. As above, so below: if it’s bright in the sky, it may just be bright in your life! By the same token, not every star can be seen in every location. Those born and living in Australia can’t see all the same stars we can see in Seattle. If it can’t be seen from your location, it’s not considered significant.
I primarily studied Brady’s Book of Fixed Stars by Bernadette Brady and was introduced to the the primary fixed stars, their positions, and their primary meanings in a natal chart. Each of the zodiacal constellations has one or more significant stars, as well as constellations such as Orion, Hercules, Andromeda, Perseus, Pegasus, Cetus the Whale, Argo the Ship, and the list goes on.
The most complex idea with using fixed stars is the notion of Parans. I’m still getting used to this one, but this is basically the relationship between a planet and a fixed star. Since earth spins around, we get to see, more or less, all the way around the sky. So, as we spin, any given star may seem to rise, culminate (be directly overhead) and set, at least, that’s how it appears to us as we watch it seem to move around us. For example, let’s say you live in Seattle, which is about 47° latitude. That means that the fixed star, Sirius, looks to you like it rises in the middle of Leo. If you had a planet in your natal chart, say Mercury, in the exact same place in the middle of Leo as Sirius is when it rises, then the planet Mercury and the fixed star Sirius have a paran relationship to each other in your natal chart. Knowledge about Sirius and its symbolic meaning in your life will go hand in hand with your knowledge about Mercury and its symbolic meaning in your life.
For more on fixed stars: