Fault lines are fractures in the Earth's crust where tectonic plates meet. These lines are crucial because they indicate areas where stress builds up due to the movement of the plates. When the stress becomes too great, it can cause an earthquake, releasing energy and shaking the ground. In addition to earthquakes, fault lines can also create unique geological features. For example, the San Andreas Fault in California is famous for its role in seismic activity. Understanding fault lines helps scientists predict potential earthquakes and assess risks in populated areas, making it an important aspect of geology and safety planning.