A metaphore used to express death. When you are cremated into ashes and your loved ones release your ashes into the wind. But you're not supposed to picture it that way; you picture someone you know who is dead and vizualize them standing untill his/her bodie truns to ash and blows away.
An example from 2pac's "Keep Ya Head Up." I'm Tryin' to find my friends, but their "Blowin' In The Wind." Meaning that they're dieing off..