Jesus speaks about the "heart" as if it were a container that a person fills with good or evil things. This is a metaphor that means what a person says reveals what the person is truly like. If you want to keep this imagery, see the UDB. You can also translate the literal meaning. Alternate translation: "A man who is truly good will speak good things, and the man who is truly evil will speak evil things" (See: [[rc://en/ta/man/translate/figs-metaphor]])