Let me use the phrase in another sentence for you. Let's say that all the local towns sets people to a big meeting. If one town sent 50 people, another sent 60 people, but your town only sent two people, you could say that your town was "poorly represented."
The example you presented means that there are lots of fossils from history, but only a few from that specific period. It is the "most poorly represented period" in history.
Does that help?