Faithfulness is spoken of as if it was an animal that would be strengthened by feeding in good pasture. AT: "nourish faithfulness" or "increase your faithfulness" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Here "heart" represents the person's inner being and thoughts. AT: "your deepest, inner desires" or "the things that you desire the most" (See: [[rc://en/ta/man/translate/figs-metonymy]])