Spirit is a brand new resource in Path of Exile 2. By default, reservation skills such as Auras, now called Persistent Buffs, will reserve Spirit.
This is another massive change going from Path of Exile 1 to Path of Exile 2, as all characters will now have a full mana pool by default, regardless of how many reservation abilities they're using.
You can view the Spirit bar just above your Mana Pool as a small cream colored border. You can hover over your Mana Pool to see how much Spirit you have/how much you are currently using.
You can also go into the Interface settings and click "Show Life/Mana Levels" which will display your Life/Mana/Spirit at all times.
Spirit is largely obtained from the campaign and items. We'll cover how to get Spirit, but first let's briefly touch on how to use Spirit.