Created in Earth's ancient past, the Planet Tablets serve as a mean for their user to gain access to their respective continent's Gaia Temple by having them unlock the portals (called the Boss Gate) inside their respective Gaia Gate. Once unlocked, these portals can take Sonic to the vicinity of the temple, which is guarded by the level's boss, and that Sonic must defeat to enter the temple. While there are seven Temples of Gaia in the world, only six keys are needed to be collected since no Gaia Gate is needed for Sonic to reach the Gaia Temple in Eggmanland, which is located near the end of the main stage.
Each Planet Tablet is split into two halves; one daytime half (the one with the red jewel) and one nighttime half (the one with the blue jewel). One of each type are needed to form one Planet Tablet, and both halves has to originate from the same continent in order to form the Planet Tablet.
In order to unlock the portal and face the boss, Sonic must collect each halves of the Planet Tablet and reassemble it. Each half is obtained after Sonic clear the main stage on each continent, with the daytime half found in the main Daytime Stage and the nighttime half found in the main Nighttime Stage. Once Sonic has both halves, he then has to seek out the guardian of the local Gaia Temple, or someone associated with them, as these people are the only ones who can assemble the two halves into a Planet Tablet.