The tablet is a slab of black marble with a hexagram engraved on it. It was formerly in the possession of Alice Hartley, before the boys stole it to cast magic. It is the equivalent of the talisman, only it's magic is not as powerful.
The tablet first appears when the boys are given the tablet to help cure their mothers of the "magic allergy" that had befallen them. The boys lay items belonging to their mothers onto the tablet that Phoebe brings out while Felix casts the spell.
After Alice destroys the talisman, the Nowhere Boys no longer have a means of channeling their magic, so they steal Alice's tablet as a replacement. After Roland witnesses magic, the boys come up with an idea of using a spell to erase his memory.