The alveolar ridge is a hard ridge found just behind the upper teeth.
Many consonant sounds are made with the tongue making contact with the alveolar ridge, such as [d, n, t, l].
To feel your alveolar ridge, put your tongue on your top teeth then pull it back along until it touches the roof of your mouth (palate). You should feel a ridge between your teeth and the palate.
Retroflex sounds are made at the alveolar ridge also, but use the underside of the tongue to make contact.