Tuner’s accuracy is a component that is measured in Cents; 100 Cents = 1 Semitone (half step).
GuitarTuna allows you to tune your instrument with the accuracy of +/- 1 cent when using the Pro accuracy feature. When this feature is disabled, the number on the screen when tuning is equal to 10 cents (1 = 1-10 cents; 2 = 11-20 cents; etc.).
To enable it, tap on Settings (cogwheel in the top left corner) → Pro accuracy. Toggle the slider so that it turns green.