One of the challenges of science is that many things in the world aren’t visible. We observe the world and find many things, but how do we know what other things are there. This is where explanation comes in. When we try to explain the changes we see in the world we need to posit a variety of other entities. We trust the existence of these entities because of the explanatory powers of our theories.
The problem with this is that since we don’t directly observe them, our conviction in their existence is only as strong as our theories. When our theoretical explanations change, we will add and subtract to our collection of things which we believe exist. For example the Rambam’s astronomy demanded the existence of transparent spheres in order to explain the motion of the stars. We no longer believe that these spheres exist, but we are forced to accept a whole new collection of invisible entities as part of our view of the world.
Our theories change our view of things in the world in two ways. They sometimes add new things, and sometimes they change our understanding of the makeup of things we know exist (for example, atomic theory accepts the existence of tables, but claims that they are mostly empty space).
The specific content of theories change, but problems at the root of things regularly change.