This is a jig for making sure your hand saw stands perpendicular to the board. It's made out of PCL plastic, a hand-moldable plastic that softens when placed in boiling water.
Nothing about this jig requires precision — the bottom doesn't have to be perfectly flat, and the slot doesn't have to be at exactly 90°. The only thing that matters is that the bottom of the try-square is in direct contact with the board.
To use it, press the bottom of the try-square up against the saw blade, and then while sawing, try to minimize the gap at the top of the saw.