This is a chipboard lid sized to fit the 4x6 Shadowbox (shown with the shadowbox in some of the photos; the shadowbox is sold separately and is linked below).
Similar to the Taj Mahal Window Card, it has a window cut-out on the front (in this version with scored sides to fold down and form the 3D box lid).
Just fold on the scored lines (make nice sharp folds before gluing the corner tabs).
Design Team Member Rhonda Secrist Thomas made both a 3D shadowbox and a window card using the same collage sheet image (but with different embellishments). You can see the 3/4 view on the far right of the wide photo (below) to see the depth of the 2 options.. More info in the blog and/or see the supply list.