The Fall of The Middle Kingdom
Common - Essentially, the ‘Common’ referred to in the PHB is an umbrella term for the languages and dialects used by the humans of today’s Middle Kingdom. Proficiency in ‘Common’ on a player’s character sheet basically means that they are able to speak the language or dialect of their region. Beyond that, it becomes trickier. Whilst broadly speaking everyone in the Middle Kingdom will be speaking the same language, the dialects vary enough that mutual comprehension is far from guaranteed.
A closer frame of reference would be to think of having Common as like having ‘European’, with specific proficiency in one region.