Crisis (クライシス Kuraishisu) is the condition required by Player Extra in order to manually use it.
When the Crisis condition is fulfilled while the Player Extra is still stacked face-down behind the Player card (usually by taking damage), the player has the option to reveal the Player Extra and put it stacked face-up on the top of their Player card per Rule Effect. If they choose not to, they won't have the chance to do it until the Crisis condition is fulfilled again.
Some Player Extra may have a card name or Player Name written in its Crisis condition (before the colon). In that case, the player need to control face-up Player card with that card name or Player Name as additional condition to use the Player Extra.