Index fossils are fossils of common and widely distributed species that can be used to date unknown fossils.
Index fossils are very useful in paleontology. These fossils are fossils of commonly found creatures that were widely distributed but unique to a specific time period.
For example, ammonites are index fossils. They were found in many places during the Mesozoic Era (245 - 65 million years ago) but went extinct in the K-T extinction event 65 million years ago. Therefore, if a fossil sample has ammonite fossils in it, it can be reasonably assumed that any other unknown fossils in the same sample came from the same era as the index fossils.