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 noun "fullness" can be stated with the verb "fills." Alternate translation: "everything that fills it" (See: [[rc://en/ta/man/jit/figs-abstractnouns]])