I am thy servant - See the notes at Psalms 116:16.
Give me understanding, that I may know thy testimonies - Since I am thy servant, instruct me in the knowledge of thy will. As I desire to obey thee, show me what will be acceptable obedience, or what thou dost require in order to acceptable service. This is a prayer of piety. A man who sincerely desires to obey God will make it a first point to ascertain what is his will, or what will constitute true obedience.