“understanding.” The Hebrew word is tabun (#08394 תָּבוּן ) (fem., tebunah)
“expressing his own mind.” In the Hebrew culture and idiom, “heart” referred to the seat of thinking and knowing, not the seat of emotion like it does in today’s English culture. This verse is not saying that a fool “expresses his heart” in the modern sense of the phrase, which is to express the deep feelings and emotions inside you. In this context is simply means to speak what is on one’s mind. This phrase has been translated as “airing his own opinion” (NIV); “revealing his own mind” (NASB). The point is that the fool is not interested in understanding what others think, he is interested in telling others what he thinks.
[For more on the Hebrew word “heart,” see commentary on Prov. 15:21.]