A short distance behind the upper teeth there is a change in the angle of the roof of the mouth called the alveolar ridge. In some people the change is quite abrupt, in other very slight.
Sounds which involve the area between the upper teeth and the alveolar ridge are called alveolars.
The word "alveolar" can be pronounced [ˌælˈvilɹ̩] or [ˌælviˈolɹ̩]. (I prefer the first.)