Freespawn is a loose term referring to units you get for free; as opposed to units recruited from provinces that have a cost in gold or units summoned through rituals that have a cost in gems.

They're useful because they're usually free of upkeep, allowing players to scale up their production without being capped by resource availability. They might come from national abilities and help define their playstyle, such as with Lemuria or Therodos, or be generated by units carrying a number of possible Summoner attributes, like with EA Pangaea's Pan mage.