Mega Evolution--which, at one time, drastically changed the playing field following its debut in Pokemon X and Y--makes a comeback, albeit a humble and not so triumphant one. This time, the Mega Ring required for Mega Evolution is only accessible after completing the main story.
Like in the past, you can make certain Pokemon "Mega Evolve" in battle, increasing their abilities. However, unlike traditional evolution, this change is not permanent and Mega Evolved Pokemon return to their prior form after battle or after being KO'd.
Preparations for Mega Evolution
Before a Pokemon can Mega Evolve, you must make the Pokemon hold their unique Mega Stone. For instance, Alakazam must hold the Alakazite. Mega Stones can be acquired by exchanging BP earned from the Battle Tree or other means.
Once the conditions are met, you can make a Pokemon Mega Evolve by selecting (or tapping) the big "Mega Evolve" button towards the left side of the bottom (touch) screen while selecting a Pokemon's moves. After that, you may proceed to select a move.
Your Pokemon will immediately Mega Evolve during the next turn, before using the move that you selected.
Mega Evolution Rule
Just like Z-Moves, you can only utilize Mega Evolution once during an entire battle--and this applies for your entire party.
In addition, since Z-Moves and Mega Evolution both require a hold item, a single Pokemon can't Mega Evolve and use a Z-Move at the same time. It's one or the other!
Although Z-Moves and Mega Evolution share some similarities, their uses are profoundly different. Most notably, Mega Evolution physically changes a Pokemon's stats and can even change their type(s) and ability.
Therefore, sometimes it can be beneficial to Mega Evolve as early as possible to immediately buff up your Pokemon or take advantage of a superior typing or ability.
Furthermore, if a Mega Evolved Pokemon looks to be at an disadvantage, you can consider switching the Pokemon out--they will still remain Mega Evolved when you bring them back in. Or you may wish to delay Mega Evolution until the threat is clear.