One who degrades another person is a fool, and a man of understanding will make himself deaf to his words(Proverbs 11:12).
When people feel good about themselves, they have no need to enhance their self-evaluation by berating others. Those who do so are exposing their own poor self-worth and to what extremes they will go in order to achieve any feeling of worth.
Solomon points out that the one who listens to such prattle is no better than the speaker. Why would anyone waste time listening to such gossip and slander unless it served some purpose? A person with good self-esteem would turn a deaf ear to such talk. Furthermore, one who listens to gossip provides the talker with an audience, thereby actually encouraging more gossip.
Solomon calls a wise person "a man of understanding." The wisdom here consists of understanding the psychology of gossips. They need to berate others for their own self-worth, and they are not above lying to disparage others. You can be certain that the person who speaks critically about someone else to you will eventually speak critically about you to someone else. The only approach, therefore, is to completely shun a gossip.
In his epochal work on lashon hara (gossip), the Chofetz Chaim states that the transgression of listening to lashon hara is every bit as serious as speaking it. If someone tries to make a listener out of you, leave, or at least politely say that you are not interested in the subject.