Prankster increases the priority of status moves by one. A move having its priority increased by Prankster does not cause it to be blocked by Quick Guard.

Does prankster affect taunt?

No, not unless the Pokemon using it also has Prankster, and is faster.

What does prankster ability do in Pokemon?

Prankster increases the priority of status moves by +1. This means that a slower Pokémon with Prankster can use a status move before the opponent uses a regular move. Or if the ability-bearer is faster, it could use a status move before the opponent uses a priority move like Aqua Jet.