Springs are the Natural Resource responsible for spawning new units. A spring is naturally unclaimed, and requires claiming by placing a Claim on it in order to spawn dwarves. Goblins will also claim springs by placing a Goblin Claim on it.
During this conflict between dwarves and goblins, springs have become a territorial focus, since they are required for building forces.
For more about how to spawn dwarves, see the Claim page.