Here the word "heart" represents a person's mind and thoughts. Alternate translation: "Keep your mind safe and guard your thoughts" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The word "it" refers to the heart, which is a metonym for the mind and thoughts. The writer speaks of a person's life as if it were a flowing spring that originates from the heart. Alternate translation: "from your thoughts comes everything you say and do" or "your thoughts determine your course of life" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-metaphor]])