John obtains a blank Tablet (left) from a Parcel Pyxis (center).
A Tablet is used with the Parcel Pyxis in the Land of Wind and Shade in order to request an item. A picture is carved onto a tablet, which is placed in a pipe to request the item. The tablets resemble Captchalogue Cards that have been used by each of the children.
John Egbert finds several carved Tablets in the introduction to Act 4, as well as a single uncarved one and a chisel. John later uses the blank tablet to request an item; specifically, his missing Sburb disc. Oddly enough, many of the Tablets seem to request items that in fact belong to John's Dad; for example, a shoe, or his hat. John, following the rules of the Parcel Pyxis, places both of these requested items within the pipes.
The tablet finds its way to the Parcel Mistress who makes it her mission to retrieve the disc from Authority Regulator. She uses the tablet to obtain the Sburb disc from the Authority Regulator and immediately posts it back in the Parcel Pyxis. It eventually arrives at John's location just after he lands on the planet again.
Rose Lalonde finds a gigantic green tablet residing in the center of a turtle building on her planet. Etched into the surface of the tablet is a picture of the Green Sun. The tablet has the same worn stone effect as John's small grey tablets. If she re-sized this tablet she may be able to use it to create a small green sun.