Albert Barnes Commentary 2 Samuel 9:6

"And Mephibosheth, the son of Jonathan, the son of Saul, came unto David, and fell on his face, and did obeisance. And David said, Mephibosheth. And he answered, Behold, thy servant!" — 2 Samuel 9:6 (ASV)

Mephibosheth - Also called Merib-baal (and Meri-baal, probably by a clerical error, 1 Chronicles 9:40). The two names seem to have the same meaning: "Bosheth," meaning "shame," is the equivalent of "Baal," and "Mephi" (scattering or destroying) is equivalent to "Merib" (contending with). Compare Ish-bosheth and Esh-baal, Jerub-baal and Jerub-besheth.

He fell on his face - He did this in fear. Such generosity to a fallen rival, as David showed by restoring his paternal property to him, seemed scarcely credible.