Proverbs 10:21 New English Translation

²¹The teaching^[a] of the righteous feeds^[b] many,
but fools die^[c] for lack of sense.^[d]

Proverbs 10:21 tn Heb “lips.” The term “lips” functions as a metonymy of cause for what is said (or in this case taught).
Proverbs 10:21 tn The verb רָעָה (raʿah) means “to feed” or “to shepherd” (e.g., Gen 48:15). What they say will meet the needs of many.
Proverbs 10:21 tn In what sense the fool “dies” is unclear. Fools ruin their lives and the lives of others by their lack of discipline and knowledge. The contrast is between enhancing life and ruining life.
Proverbs 10:21 tn The term לֵב (lev, “mind, heart”) is used as a metonymy of association for what one does with the mind (i.e., thinking), and so refers to discernment, wisdom, good sense.

New English Translation (NET)