Relative age is determining how old something is relative to something else. So say scientists have a rock that was found deep in a fault. They can then assume that anything above that rock is newer than that, so therefore younger. So any fault will only be as old, relatively speaking, to the deepest rock they find. However, due to plate shifts and other seismic activity, these results won't be accurate, carbon dating is much more accurate but takes a lot of work.