The machine is used for the first time, drilling holes in it's own base.
To allow various clamps and other fixtures to be securely attached to the table, a grid of holes is drilled in the table. Metal inserts are then installed in the holes, in this case type D inserts, 20mm length, threaded M6.
The holes go right through the table, so that dust will not collect in them.
Any M6 threaded screw can be fitted into the inserts, such as threaded rod, handwheels or bolts, so that clamps, vices and other items can be used to secure workpieces to the table.