The righteousness of thy testimonies - The principles of righteousness on which they are founded. Those testimonies - those laws - are not arbitrary, or the mere expressions of will. They are founded on right and justice as seen by God, and his laws are his testimony as to what truth and justice are.
Is everlasting - See the notes at Psalms 119:142.
Give me understanding, and I shall live - Give me a right view of thy law, and thy truth, and I shall have real life. See the notes at Psalms 119:34.