Summon means 'enter the realm,' for minions. When you cast a minion spell, the result is that you summon that minion, following normal restrictions about casting spells. When another effect causes you to summon a minion directly without casting it, e.g. Mother Nature, Brother Knight, Knighthood, etc., you simply put it into the realm. You don't need to pay mana, meet threshold requirements, or choose a location you control, as you usually do when you cast a minion spell. Typically a summon effect will specify where the summoned minion appears. For example, Mother Nature specifies that the minion is summoned 'here,' i.e., Mother Nature's location, even if that's an enemy site. If a summon effect does not specify where the summoned minion appears, e.g. Raise Dead or Plague of Frogs, you may summon it to any location in any region. If you try to summon a minion to a location that it can't enter, e.g. Gnome Hollows, it doesn't change zones and remains where it was.