Complex sentence: When a Hatchery or Creep Colony (and variations) is built, it slowly generates Creep around it (stopping when a certain radius is reached), allowing buildings to grow there (remember that Zerg buildings are living organisms).
Split into simpler sentences: When a Hatchery or Creep Colony (and variations) is built, it slowly generates Creep around it, eventually stopping when a certain radius is reached. The creep allows buildings to grow there (remember that Zerg buildings are living organisms).