How do fossils tell us about earth's history?
Fossils are a way at looking at the past by knowing what was there.
Whatever you find when digging up a fossil can tell you a lot about what was living around that age. If there was a plant then there had to be an atmosphere and elements to keep that plant alive. If there was some sort of animal and depending on what the animal looks like you can determine what the animal ate and how it used its features to live. By doing this and with enough information you can create a map of what the time must have looked like even if you can't see it with your own eyes.
I hope this helps, but I am in no way an expert!