Saul was proceeding along one side of the mountain, and David and his men along the other side. Even though David was hurrying to get away, Saul and his men were closing in on David and his men to capture them.
Here, 1. The Ziphites offer their service to Saul, to betray David to him, Sa1 23:19, Sa1 23:20. He was sheltering himself in the wilderness of Ziph (Sa1 23:14, Sa1 23:15), putting the more confidence in the people of that country because they were of his own tribe.
Commenting on 1 Samuel 23:19-29
And Saul went on this side of the mountain, and David and his men on that side of the mountain,.... Saul with his army came to the very mountain where David and his men were, the one was on one side of it, and the other on the other side; there was only one mountain between them: and David made haste to get away for...
Saul went on this side of the mountain - Evidently not knowing that David and his men were on the other side.