Adullam, one of the royal cities of the Canaanites, now 'Aid-el-ma (Joshua 12:15 ; 15:35). It stood on the old Roman road in the valley of Elah (q.v.), which was the scene of David's memorable victory over Goliath (1 Samuel 17:2), and not far from Gath. It was one of the towns which Rehoboam fortified against Egypt (2 Chronicles 11:7). It was called "the glory of Israel" (Micah 1:15)

The Cave of Adullam has been discovered about 2 miles south of the scene of David's triumph, and 13 miles west of Bethlehem. At the place is a hill some 500 feet high pierced with numerous caverns, in one of which David gathered together "everyone in distress, and every one that was in debt, and everyone that was discontented" (1 Samuel 22:2). 

Easton's Bible Dictionary  - Adullam

It is believed that out of this group of rag-tags came David's mighty men (2 Samuel 23: 8-13). They too became giant killers just like their leader David.  David typifies our heavenly king Jesus who reigns victorious over every enemy Ephesians (1: 19-23).  His mighty men typify the New Testament overcomers who have been transformed and have taken on the likeness and image of their leader.