Major boundaries in Earth's time scale happen when there were major extinction events that wiped certain kinds of fossils out of the fossil record.
When you find the same fossils in rocks far away, you know that the sediments those rocks must have been laid down at the same time.
Conveniently, the vast majority of rocks exposed on the surface of Earth are less than a few hundred million years old, which corresponds to the time when there was abundant multicellular life here.
Look closely at the Geologic Time Scale chart, and you might notice that the first three columns don't even go back 600 million years.
The more fossils you find at a location, the more you can fine-tune the relative age of this layer versus that layer.
Of course, this only works for rocks that contain abundant fossils.
Unfortunately, those methods don't work on all rocks, and they don't work at all if you don't have rocks in the laboratory to age-date. They are descriptions of how one rock or event is older or younger than another.