Here's how it works:

* Superposition: The principle of superposition states that in undisturbed rock sequences, the oldest layers are at the bottom and the youngest layers are at the top.

* Correlation: Geologists compare rock layers from different locations based on their composition, fossils, and other features. They try to match up layers that formed at the same time.

* Relative Dating: This refers to the determination of the age of a rock layer relative to other rock layers, without knowing the exact numerical age.

* Index Fossils: Fossils of organisms that lived for a short period of time and were geographically widespread are called index fossils. They are useful for correlating rock layers across different locations.

Stratigraphy helps us understand:

* Geological History: The sequence of events that formed the rocks in a region.

* Past Environments: The types of environments that existed in the past, based on the fossils and sedimentary structures in the rocks.

* Resources: The location of mineral and energy resources, which are often associated with specific rock types and geological events.

Let me know if you have any other questions!