A step-by-step guide on how to perform this procedure with the Accessport can be found here: How To: Perform PDK Clutch Re-Learn
Over time clutches can wear and the thickness can decrease. Performing a PDK relearn will help the PDK verify clutch thickness and position. This is particularly useful after a clutch replacement or in cars with higher mileage.
The adaption process an be quite loud and the car may clunk or lurch somewhat violently. This is normal during the adaption process.