How are the extra play in games going to work for this years NCAA men's basketball tourney?

I know there are 68 teams. Why do they call them 68 teams, when technically it's still gong to be 64 for the big Tournament? Will one set of teams play the other set, than move on for one more final play in game, and the winner of that will make the 64th team in the NCAA tourney? How are the play in game teams going to be ceeded in bracketology? And how is all this going to work and how are the ceedings going to work with the extra play in games added? Any info on how this will be played out and why are they adding teams this year will be helpful.