This big chunk of metal is used to improve the print quality of your Anycubic Photon when combined with good quality linear rails (not included, check them out on Amazon). See the video at the bottom of this page for more details if you haven’t already. Also if you’re a Patreon supporter and just want the 3D model, go here.
Precision machined from a solid billet of 6061 aluminum (or Al-you-minimum for our European friends), and then anodized by two idiots in their garage. I think we did an alright job though, and each one was tested against our printer to make sure it still had a good snug fit.