It means that even if two things seem different to each other, in essence they are still the same thing. The phrase originates from the different pronunciation of potato: potato, potahto. But it doesn't matter because it refers to the same thing. In that context, there is a running joke in the series making fun of Ross and his interest in studying dinosaurs. Basically, it means there is no difference between dinosaurs and dragons (dragons, being a bit mythical). It's supposed to be funny.