Purifying Pokemon can be a confusing process for trainers new to the series in Pokemon GO.
Shadow Pokemon were originally introduced in the early 2000's with GameCube titles such as Pokemon XD: Gale of Darkness and Pokemon Colosseum. Innocent Pokemon underwent a process to make them hateful and aggressive, corrupting them into Shadow Pokemon that were twice as strong at the cost of their free will and resistance.
Fortunately, thanks to courageous trainers, these Pokemon can be returned to their original forms.
What Does Purifying a Pokemon Do in Pokemon GO?
Shadow Pokemon were introduced alongside the appearance of Team GO Rocket—Pokemon GO's very own Team Rocket splinter group. Rocket Grunts and Executives use Shadow Pokemon in battle when defending captured PokeStops. After their defeat, trainers have the opportunity to catch a Shadow Pokemon from their opponent's team.
Essentially, "purifying" a Pokemon returns it to its natural state before Team GO Rocket mutated them into Shadow Pokemon.
The logistics of the process are simple. While living as a Shadow Pokemon, the affected creature is much stronger and costs less candy and stardust to power-up. However, this Pokemon's defense drops significantly and is more prone to super-effective attacks from other Shadow Pokemon.
It takes a certain amount of candy and stardust to purify a Shadow Pokemon. Once purified, the Pokemon's CP will return to what it would've been before it was transformed and it will have the charged move "Return"—which does more damage based on the strength of your relationship with it.