Relative dating cannot tell us the actual age of a rock; it can only tell us whether one rock is older or younger than another.The most common form of relative dating is called stratigraphic succession.Paul is super awesome, so I'm going to take him at his word.
Some fossils, called index fossils, are particularly useful in correlating rocks.
For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic.
Some of the most useful fossils for dating purposes are very small ones.
For example, microscopic dinoflagellates have been studied and dated in great detail around the world.
Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence.