Polystrate fossils are fossils that extend through multiple layers of sedimentary rock, often seen in the form of tree trunks or other plant remains. These fossils provide evidence that the sedimentary layers were deposited rapidly, rather than over long geological periods, as the organisms would have decayed if the layers formed slowly. Typically found in coal deposits or riverbanks, polystrate fossils can help scientists understand past environments and the processes that led to fossilization. They are important for studying the geological history of an area and can indicate events like floods or rapid sedimentation that preserved the organisms.