She was one of the wives of King David in the Old Testament. See http://saints.sqpn.com/abigail-the-matriarch/
, which includes an excerpt from the First Book of Samuel that mentions Abigail.
I also found this brief description on another website:
Abigail was the wife of Nabal the Carmelite. And after his death, she became the wife of David (1 Samuel 25:3, 14-42; 2 Samuel 2:2; 1 Chronicles 3:1).
Abigail's husband, Nabal, was an ill-tempered, drunken man. When David was hiding from the jealous King Saul, he asked Nabal for food for himself and his men. Nabal bluntly refused. Abigail, in her wisdom, gathered enough food for David's men, rode out to meet David, and bowed before him to show her respect.
By agreeing with David that Nabal had acted with great disrespect, she stemmed David's anger. To Abigail's credit, she did not leave her Godless husband. When Nabal died, David married Abigail and she later bore him a son, Chileab.
Many of the holy men and women of the Old Testament are considered saints even though they lived long before the Church had a formal canonization process.