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. , , , 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, 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. or , 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. , it doesn't change zones and remains where it was.