Have you seen Forrest Gump? In the film Forrest says "My momma always said "Life is like a box of chocolates. you never know what you're gonna get." "The future is open" means something like this. It is always full of surprises.

It means that the future is open, full of surprises and you'll never know what happens next.

"The future is an open book" It means that the future has a lot of possibilities. Anything can happen. It depends on the choices we make and how we want our future to be.