'Service' in the Bible

Now there was a servant from Saul's house named Ziba, so he was summoned to David. The king asked him, "Are you Ziba?" He replied, "At your service."

When Mephibosheth son of Jonathan, the son of Saul, came to David, he bowed low with his face toward the ground. David said, "Mephibosheth?" He replied, "Yes, at your service."