These are brackets that slide into the T-slots of the aluminum bed of the Shopbot Desktop (and likely other Shopbot tools and many other CNCs). It allows you to then craft a thin ply, lexan, whatever material you want shield that you've glued the "feet" parts onto so allow it to slide onto the bracket. This makes for a quick and easy attach/remove guard to keep dust and chips off of your rails thus reducing the risk of errors and increasing the longevity of your bearings.
I am putting 4 brackets on each side of my table (18" length there) so I will print 8 brackets, 8 washers (adjust the height of these to accommodate whatever screw heights you have), and 16 feet to glue to the actual shield itself.
On top of this I will use one machine screw and one T-nut as seen in the pictures per bracket.