Spirit is the resource used by the Druid's skills.
Spirit doesn’t drain when not in use, it only drains when the Druid uses skills that use it.
Spirit can be generated by using skills that refill the Spirit orb. Basic Skills such as Earthspike, Claw, Storm Strike, Maul, and Windsheer all generate spirit.
Check the Druid Skill tree to see all skill that generate Spirit.