Sorry, Im kind of new to this and i dont understand all the terms. Im not sure what the alpha channel is, do all graphics clips have an alpha channel?

Hello all, Ive just recently moved over to Resolume Arena from GrandVJ and I like it alot. As with transitions there are alot of new ways Im learning to do the things I did in GrandVJ. One thing I cannot figure out for the life of me is Rejecting Black or Reject White, or any color for that matter. ...