Biostratigraphy is a branch of geology that uses fossils to date and correlate rock layers. By studying the distribution of different fossilized organisms within sedimentary rocks, scientists can determine the relative ages of these layers. This helps in understanding the geological history of an area and the evolution of life over time. The technique relies on the principle that certain fossils, known as index fossils, are characteristic of specific geological periods. By identifying these fossils, geologists can establish a timeline and make connections between rock formations in different locations, aiding in the reconstruction of past environments and ecosystems.