Keys are a mechanism for players to earn experience by acting in a certain manner. They tend to take the following form (taken from Lady Blackbird):

Key of the Impostor

You are in disguise, passing yourself off as commoner. Turn this key when you perform well enough to fool someone with your disguise. Buyoff: Reveal your true identity to someone you fooled.

Turning a key (also sometimes called hitting a key), earns the player experience points.

Keys often feature a buyoff, which is a condition that allows the player to remove the key and replace it with another. This action also often earns experience points, but more importantly, it allows the character to match the player’s vision and represents character growth.