lol, they say that because when something is at the rumor phase no one knows about it yet and those that do know aren't brave enough to make the move. By the time it hits the news and becomes official everyone and their mother starts buying and it allows the rumor buyers to start closing out positions. When you sell, you have to sell while its still moving up because thats a guaranteed buyer for your shares.
p.s. normally i would have made fun of you for asking that question but i remember you mentioned yesterday that you are only 19 so i will easy on you. i wish i had someone to explain this crap to me when i was your age. I had to lose ALOTTT of money to know what I know now.