Tuner Monsters are required for the Synchro Summon of a Synchro Monster. When the total Level of the Tuner monster and the other monsters on your side of the field matches the Synchro monster's Level, by sending them to the Graveyard, you can Summon the Synchro Monster.
Tuner Monsters are the first secondary Type for Normal Monster. They can also be another Type of Effect Monsters, like Union and Spirit. Tuner Monsters can be Normal, Effect, or even Synchro Monsters.