A billboard projects a message into the minds of others and in dreams symbolizes that you have the power to project your mind. This can be used for a number of things.
The first is that you can use your strong intellect to rise mentally above a situation. You don’t always do this though!
Another thing it is useful for is hypnosis, where you project your mind onto a client to help them ‘think’ their way through an issue. Literally your thoughts direct the thoughts of the client.
Another use, although controversial in some circles is absent healing. Here you project your mind onto the client and use your thoughts to think them well. The connection between the brain and the immune system has been established. Absent healing projects positive healing thoughts onto the client with the intention that the healing takes root.
Lastly this strong mind can be used to project thoughts to others, including groups of others. See flying.
