Art Cooldown. You will not find mana in Xenoblade Chronicles. All Arts are on a cooldown system in which you have to wait a certain amount of time before the ability becomes available again. Depending on the art, this could be a few seconds to over a minute. 

In the image, and at the bottom of the screen, you can see that grayed out abilities are ones on cooldown and cannot be used. Each ability has a meter that rises from bottom to top. Once the meter reaches the top the ability is ready 

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