What does it mean?
Wise people talk only when they have a real point. Foolish people talk just to fill the silence. The joke is in the word play: "have something to say" and "have to say something".
Who said it?
Plato: Ancient Greek philosopher (c. 428–348 BC).
These exact words are widely credited to Plato, but they don’t appear in their known work. They may be a later paraphrase or a misattribution.