It's a simple concept - you write an email today and schedule to send it to yourself in the future. I just did one to send to myself on my 24th birthday asking all the same questions I am asking myself now, but hopefully by then I will know the answers.
Will your dreams come true?
Will you get that job?
Will you run that marathon you always promise yourself you will one day run?
Ok, so that last one probably will never have an answer but you know...