What does "Share what I watch" do?

It lets other members see your viewing. It's off until you turn it on.

Go to Account Dashboard → Security & Privacy → Privacy. Share what I watch shows your viewing to other members: in watch parties, on your profile, and in reactions. It is off until you turn it on. The same section has Do not sell or share my personal information and Personalized ads. If your browser sends Global Privacy Control, "Do not sell or share" stays on.