How do fossils help biologists understand the history of life on earth?
They help in that they show the transition of life through the ages.
For example, they can show several ancestral species from which our present day species could have possibly descended. Also, they could also show physical features that present day animals also have.
Of course, once you get into that there is a ton of controversy you will face, but for your average high school Biology course, what I said above is good to know.
Hope that helped :)