When you strum a string, it vibrates and produces a sound wave. These waves are measured in Hz (Hertz), and each tone has its own frequency.
With GuitarTuna, you have the option to create custom pitch notations, with the Calibrate feature. Tap on Settings (cogwheel in the top left corner) in the Tuner tab. From the menu that opens, toggle on the Calibrate option so that the slider turns green.
Using ↑ (Up) or ↓ (Down) arrows, you can adjust the frequency as per your preference.
By default, the frequency is set to 440 Hz, which is recognized as an international tuning standard. Guiseppe Verdi, an Italian composer, placed A exactly at 432 Hz, which is the most well known alternative tuning.
It's really up to you to choose which frequency suits you best!