This is an assembly jig for Y motor holder for AM8 frame (Anet A8, extrusions).
It works once you have assembled (at least) the XY part of the frame (I mean, all but the gantry).
It's supposed to be placed against the rear X extrusions, aluminum corner brackets at its left, motor mount at its right. However, it's not that critical, since you can always move the mount itself along the extrusion, and/or the pulley along the shaft.
Works for 20x20x17 corner brackets.
You can find the Y motor mount here: