“Appoint steadfast love (hesed v’emet, literally “love and truth”) to guard him.” (61:8)
A hendiadys is a word pair consisting of two nouns (or sometimes a noun and an adjective) joined by a conjunction, in which the two words modify each other. It is a common Biblical form. See here for a further explanation and examples.
Hesed v’emet or hesed shel emet is a term often used to refer to the work of the Hevra Kadisha, the group that prepares bodies for burial by washing and dressing them in linen shrouds. The “truest form of love” is the love you show for a person without expectation that you will receive anything in return. This certainly applies to the mitzvah of taking care of the dead.
In the case of this Psalmist, who is referring to the circumstances of a King, hesed v’emet, steadfast love, refers to the kind of love that is shown by God to human beings, who can never hope to repay the love to a God has no real need for human favor. It is also a model for the love in a human relationship. If we qualify everything we do for someone we love with the condition that we get something in return, the relationship will inevitably deteriorate. This is as taught in Pirke Avot 5:18:
“A loving relationship which depends upon something, [when] that thing is gone, the love is gone. But [a loving relationship] which does not depend upon something will never come to an end.”