Tuner Monsters are Monsters with "Tuner" listed on the Type/Ability line. They are required for the Synchro Summon of a Synchro Monster. Normal, Fusion, Effect, Pendulum, and Synchro Monsters themselves can all be Tuner Monsters, although some Card effects can treat non-Tuner Monsters as Tuners. Flip and Union Monsters can also be Tuner Monsters.
To perform a Synchro Summon, you send Monsters you control (including a Tuner Monster) to the Graveyard whose total Level is exactly equal to that of a Synchro Monster in your Extra Deck, then you can Synchro Summon that Synchro Monster. Usually, Synchro Summons require exactly one Tuner Monster, but some Synchro Monsters allow or require multiple Tuner Monsters (An example is “Hot Red Dragon Archfiend King Calamity”).
A number of support Cards specifically deal with Tuner Monsters, including "Mind Trust", "Boost Warrior", and "Quillbolt Hedgehog", which can further aid in setting up a Synchro Summon.
Some Synchro Monsters require a specific Tuner Monster to be used ("Sun Dragon Inti"), or a Tuner of a specific Type ("Trident Dragion"), Attribute ("Dragocytos Corrupted Nethersoul Dragon"), or archetype ("Blackwing Tamer - Obsidian Hawk Joe"). Likewise, some Tuners can only be used as Synchro Material for a limited set of Synchro Monsters, such as "Brotherhood of the Fire Fist - Spirit".
Tuner Synchro Monsters
There are also Tuner Synchro Monsters, Synchro Monsters that are themselves Tuners. Due to their difficulty of use, they tend to have effects that make Synchro Summoning easier, such as by manipulating their own Level ("Phonon Pulse Dragon") or allowing you to Synchro Summon during the opponent's turn ("T.G. Wonder Magician").