index fossils are fossils of preserved remains or traces of a once living organism that lived in the specific geological past time and used to heip identify and date the age of rock layers.
The fossils of very popular forms of life during certain periods in the Earth's history.
For example the ammonites: They were very abundant in the Mesozoic and the method of dating them is by their suture. If you find a certain suture pattern in an ammonite fossil present in a rock layer you point to that rock with your index finger and announce "this layer is 85.5 m.y old" (as an example).