Mega Evolution in Pokémon Go allows your Pokémon to become even more powerful during its final evolution, and we might get an idea who will be the first capable of Mega Evolving. This is a way to make them even more powerful, but for a limited time. A Pokémon capable of Mega Evolution cannot stay in this state forever. When their cooldown expires, you must activate it again to bring it back. This is where the Mega Candy comes in and why you need to collect it.
Most of the information we have comes from the Pokeminers datamining group who posted what they learned from the Pokémon Go Code on Reddit. They encourage many of their readers to remember what they learn and what they post about can change. Their history is exceptionally rich and relevant. However, what we do know remains speculative until Niantic confirms the information.
We don't know too many details about Mega Candy beyond the fact that you'll probably need it to evolve your Pokémon into its Mega Evolution. You can receive it by completing Mega Raids, which are different from the traditional raids you see all over your town in Gyms. These have a special marker next to them to help them stand out. You can also earn Mega Candy by completing quests, probably the ones you need to unlock the ability to Mega Evolve your Pokémon.
Unlike traditional games, your Pokemon can be mega-evolved outside of combat, probably giving you almost 24 hours in this form before it vanishes. This is why your Mega Evolution has a timer, and when that timer reaches zero, they revert to their standard form. We don't know if your Pokémon has a timer until it can do another Mega Evolution, or if you have enough candy, the Mega Evolution is available immediately.
Either way, Mega Candy is a new item that any trainer interested in doing a Mega Evolution will need to collect to regularly have the strongest Pokemon on their list.
