The command M303 will heat-up your hot-end to a set-point temperature and record data as it does so. Therefore, while your hot-end is undergoing it is important to replicate the conditions that your 3D printer prints in. For example, turning on your nozzle fans. First, the tuning process will heat the hot-end to your set temperature. It will likely overshoot the set-point. The nozzle will then cool-down to room temperature. If you instructed the control board to perform multiple cycles, it will repeat this process multiple times. Once complete, the firmware will return suggested control variable values.