Ghost Pieces are special level-5 pieces. Not only are they large but they also unlock a new action you can use during your turn when you have one of the Ghost Pieces in your supply:
• Ghost Piece Action - Return a Ghost Piece to the reserve and take two or more level 1-4 pieces to your supply. The pieces you take must form the shape of the Ghost Piece you have returned. As part of this action, you may immediately place these pieces on the puzzles in front of you, but no more than one piece per a puzzle.
It is not possible to get Ghost Pieces (level 5) by using the regular upgrade action during your turn. The only way to get them is by completing the Ghost Piece expansion Puzzles.