A fossil is evidence of life from far back in the past. There are many types of fossil but the best known is bone. Bone is hard and strong and is often the only part of an animal to survive years after an animal has died. If those bones are buried, there is a chance that someone may find it millions of years later.
Other types of fossil include shells of sea animals like sea snails or oysters and clams. Plants can also be preserved and, occasionally, soft tissue like muscles, but these are very rare.