So basically, a non-activated special summon (referred to as a build-in summon, a summon that doesn't start a chain, or colloquially an inherent summon) is a summon that lacks a cost activation window. Basically, if it has a colon or semi-colon, like Rikka Princess, Dream Shark, or Spore, it is an activated effect that starts a chain and can be responded to. If it doesn't contain one, it's then known as an (unoffical name) inherent summon, with popular examples being Diabellstar the Black Witch, The Horus Monsters, or Kashtira Unicorn/Fenrir.
This is a big distinction because as I said, since it doesn't give you a response window, meaning you can't chain Maxx C or Fuwaloss to it, and most significant of all, you can't negate the activated effect to summon (but some cards can negate the summon itself, like Infernoid Flood or Black Horn of Heaven).