A mediator is a person who helps two parties come to an agreement. Christ, as mediator, made it possible for a better covenant between God and humans to exist.
This can be stated in active form. Alternate translation: "covenant. It was this covenant that God made based on better promises" or "covenant. God promised better things when he made this covenant" (See: [[rc://en/ta/man/jit/figs-activepassive]])