Fossils, as you said, are a representation of animals or plants that lived once in the past, millions of years ago, which became findable on the fossil record by different circumstances of the environment they lived, like the kind of soil or rocks, that made possible the preservation of these. But abording the very point of the question, why do fossils are important to the study of evolution, we could say that they are so because they represent former stages of too many species existing today since they were developing themselves in a different environment, and below different conditions, like the disponibility of different feeding resources, or under different environmental pressures, like ancient predators back in the day, or places they can inhabit.