Depends of the period and the part of France. But it can be really difficult to understand ! Here is an exemple with "La chanson de Guillaume". You can ask google about "langue d'oc" and "langue d'oïl" if you want more details.
French has been evolving a lot for centuries. But there was a main reform during the 18th century (1700-..) when the Académie Française published a dictionary. And the modern French we speaks was really settled during the 19th century (1800-). But the language is still evolving, the last reform was during the 90’s, not so long ago. (My reply is simply to complete that it’s hard to show main differences, because it changed in the long run through multiple reforms)