What is True-tone?
True Tone is a display technology used by Apple for adjusting the whitepoint of your screen depending on your environmental lighting. It has sensor to detect your surrounding lighting and then adjust the ambient of your screen display to suit your surrounding more. The way Apple explain it is like this "The 9.7-inch iPad Pro features advanced display technologies, including a True Tone display, which uses new four-channel sensors to dynamically adjust the white balance of the display to match the light around you for a more natural and accurate, paper-white viewing experience."
On apple device, true tone is switched on by default but you can switch it off from Settings > Display & Brightness. The checkbox for True Tone is there.