And at some point in the game you would say: tonight I am going to dream in such a way that I don't know that I'm dreaming, so that you would take the experience of the dream for complete reality.
Watts is describing a dream-game in which the dreamer deliberately chooses to forget that the experience is a dream, so that it feels like “complete reality” 1. He develops the same idea elsewhere by saying one could “forget this dream is a dream” and become “really shocked” by it 2, and that after repeated controlled dreams one might decide to “forget it’s a dream and have a real thrill” 12. He also connects this with the broader idea that dream experience can become vivid enough to seem fully real while you are inside it 610.