It's how in-Synch a Tuner and Receiver are.

Starting at zero and going to one-hundred. Zero is "no remotely positive relationship", one is "receiver desires to help Tuner" and grants the most basic level of the Tuner's abilities at their weakest stage. For weapon-type Tuners this is just the physical buffs at a low level, for example. One-hundred is full power.

As mentioned before, Synchronization Overdrive is a theoretical state of exceeding one hundred percent synchronization.