Radiometric dating is a method used to date rocks. Scientists do this by comparing the radioactive materials in the rock and the decay rate. The rocks with radioactive materials have half lives. For example, when potassium goes through its half life you get some argon. And this process keeps happening after every half life.
Index Fossils is when scientists research different types of fossils and figure out what time period they are all in and date them that way.
Relative Dating is when scientists examine different layers of rocks and determine when they were deposited. Once they figure out what age the rock layer is then they look for different fossils in those layers.