Set is when a Creature is placed vertically face-down.
You can Set a Creature from your Hand instead of Summoning it. If a Creature would require Tribute to be Summoned, then you must Tribute that number of Creatures in order to Set it.
If a Set Creature is an Attack Target, then you do NOT Summon the Creature or otherwise change its Battle Mode, except due to a Creature's Ability (including the Set Creature's). A Set Creature is placed face-up in the Grave when sent there.