Hub areas are an unofficial terminology for larger settlements such as cities from which more quests can be found, some regions have more than one hub and are encountered as the story advances. Once arriving in a new area it is not possible to leave until Luka has spoken to most (if not, all) of the townspeople to unlock all of the possible quests that branch out from the city.
It is also not possible to revisit missed quests or any previous location once Luka decide to move onto a new hub city, something Alice makes clear before he decides to move on to the next main destination. However, sometimes the storyline will force or give the option to revisit past areas, albeit in a rather limited fashion.
As quests are completed their corresponding hub will begin to advance in it's own dialogue and storyline. One such example is San Ilia, which becomes plagued with fairies and ghosts as the Forest of Spirits and Haunted Manor quests are completed.