An alveolar ridge is one of the two jaw

Jaw

The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...

 ridges either on the roof of the mouth between the upper teeth and the hard palate

Hard palate

The hard palate is a thin horizontal bony plate of the skull, located in the roof of the mouth. It spans the arch formed by the upper teeth.It is formed by the palatine process of the maxilla and horizontal plate of palatine bone....

 or on the bottom of the mouth behind the lower teeth. The alveolar ridges contain the sockets (alveoli) of the teeth.
They can be felt with the tongue

Tongue

The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...

 in the area right above the top teeth or below the bottom teeth. Its surface is covered with little ridges. Sounds made with the tongue touching the alveolar ridge while speaking are called alveolar. Examples of alveolar consonant

Alveolar consonant

Alveolar consonants are articulated with the tongue against or close to the superior alveolar ridge, which is called that because it contains the alveoli of the superior teeth...

s in English are, for instance, [t], [d], [s], [z], [n], [l] like in the words time, dragon, silly, zoo, nasty and lollipop. There are exceptions to this however, such as speakers of the New York Accent who pronounce [t] and [d] at the back of their teeth. When pronouncing these sounds the tongue touches ([t], [d], [n]), or nearly touches ([s], [z]) the upper alveolar ridge which can also be referred to as gum ridge. In many other languages these same consonants are articulated slightly differently, and are often described as dental consonants. In many languages consonants are articulated with the tongue touching or close to the upper alveolar ridge. The former are called alveolar plosives, and the latter alveolar fricatives.