Hutton's law, also known as the principle of cross-cutting relationships, states that if a geologic feature cuts across another, the feature that has been cut is older than the feature that is doing the cutting. This principle is a fundamental aspect of relative dating in geology.
When a fault or igneous intrusion occurs, it disrupts the existing layers of rock or structures. Therefore, it is concluded that the fault or intrusion must be younger than the rocks that it cuts through. This means that if you encounter a fault in a sequence of rock layers, the rock layers that it disrupts were present before the fault was formed. Thus, the correct understanding is that the fault is younger than any rock it cuts through.
This principle allows geologists to create a chronological sequence of events in the geological history of an area, which is essential for understanding the age and evolution of the Earth's surface.