A Summon is the act of putting a creature from your hand into the battle zone.

To summon a creature from your hand, you check its mana cost first. The mana cost for a creature is in the upper left corner of the card. You pay a creature's cost by tapping cards in your mana zone for mana. At least 1 of the tapped mana must be of the same civilization as the creature. If you are unable to pay for the cost of it, you can't summon the creature.

After you pay the cost, put it into the battle zone. There's no limit to the number of creatures you can have in the battle zone. You can only summon creatures on your turn, and only if none of your creatures have attacked yet. Creatures can't attack on the turn you put them into the battle zone (unless an effect like "Speed attacker" says so or if the creature is an evolution creature or linked God), because they have summoning sickness. They will be able to attack on your next turn.

Summoning specifically refers to putting a creature into the battle zone from your hand by paying its mana cost, or abilities that specifically say that they summon, like Gravity Zero, Ninja Strike, and Mana Reburst. Effects that put a creature into the battle zone that do not say that they summon, like Hyperspatial spells and Soulswap, do not summon creatures, and the creatures that are put into the battle zone from cards like these are not treated as being summoned.

There are cards that allow you to summon creatures out of the mana zone or graveyard such as Beautiful Fairy Minmei, Hyper Overthrow, Nation's Strongest Sumo, and Hades, Emperor of Death.