OK, I did a quick Google, and here's what I found
A waiver is a permission granted to a Club that desires to assign or release one of its players. There are basically two types of waivers -- waivers for the assignment of a player and waivers for the unconditional release of a player. In both cases, waivers are granted only after all the other Major League Clubs have been given an opportunity to claim the player and none has done so. With regard to assignment waivers, permission is granted for a specific period of time. With unconditional release waivers, once permission is granted the player is a free agent.