This is a superscription that tells about the psalm. Some scholars say that this is part of the scripture and some say that it is not. (See "What are Superscriptions in Psalms" in [Introduction to Psalms](../front/intro.md).)
The abstract nouns "faithfulness" and "justice" can be translated with adjectives. Alternate translation: "I will sing that you are faithful to your covenant and that you are just" (See: [[rc://en/ta/man/jit/figs-abstractnouns]])