Here the word "heart" represents the person's mind and thoughts. AT: "A person makes plans in his mind" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Possible meanings are 1) Yahweh speaks his answer to a person's plans, which is a metaphor meaning that Yahweh determines the outcome of that person's plans or 2) Yahweh enables a person to speak words about the plans that he has made. (See: [[rc://en/ta/man/translate/figs-metaphor]])