John Gill Commentary Psalms 119:125

"I am thy servant; give me understanding, That I may know thy testimonies." — Psalms 119:125 (ASV)

I [am] your servant
Not only by creation, but by grace; and as he had a work to do, he desires to know what it was; and as it was proper he should know his Master's will, he applies to him for it; using this as an argument, that he was his servant, devoted to his service, and willing to perform it to the best of his knowledge and ability; and therefore prays,

give me understanding, that I may know your testimonies ;
the Scriptures, which testify of the will of God; which are only rightly understood by those who have their understandings opened and enlightened; or have an understanding given them, that they may understand them, so as to receive and embrace the doctrine, and do the precepts of them: and such an understanding is the gift of God, and owing to his powerful and efficacious grace; see (Luke 24:45) (1 John 5:20) .