Tuner

A Tuner monster is required for a Synchro Summon. When the combined Levels of a Tuner monster and 1 or more non-Tuner monsters on your side of the field exactly equal the Level of a Synchro Monster in your Extra Deck, you can send those monsters to the Graveyard to Summon that Synchro Monster.

Some Synchro Monsters require a specific Tuner monster such as Junk Warrior, which can only be Synchro Summoned using Junk Synchron as one of the Synchro Material Monsters.