They are abundant, O king, that can always speak agreeable words. The speaker, however, is rare, as also the hearer, of words that are disagreeable but medicinal. That man who, without regarding what is agreeable or disagreeable to his master but keeping virtue alone in view, sayeth what is unpalatable, but medicinal, truly addeth to the strength of the king.

In plain words: Vidura says many people can always say pleasant things, but it is rare to find someone who speaks hard truths that help, and rare to find someone willing to hear them. A servant who tells his master an unwelcome but helpful truth, caring only about what is right, truly strengthens the king.