Pop rocks are mostly made of sugar and carbon dioxide. The sugar is melted down with flavoring and mixed with carbon dioxide gas. When this mixture cools, the candy hardens and traps the carbon dioxide inside. When you eat the candy, your saliva dissolves the sugar, releasing the carbon dioxide, exactly like what happens when you open a can of soda pop. The only difference between soda pop and pop rocks is that in soda, the carbon dioxide is contained in a liquid, whereas in pop rocks the carbon dioxide is contained in a solid. The reaction going on in your mouth is the same!

Pop rocks are just made of a few ingredients (like sugars) that are melted together. The reason they "pop" in your mouth is because when the sugars are melted together at higher temperatures, they apply a high pressure of carbon dioxide, that gets trapped inside when the candy cools. When the candy dissolves in your mouth, the trapped carbon dioxide "pops" and is released. This is the same thing that happens with pop or soda, where a high pressure of carbon dioxide is applied to the drink, which traps CO₂ inside the water.