Optimum braking conditions are when the pad and the rim are in direct contact, with no liquid nor dust in-between.
Unfortunately, in real life, various particles, liquids or grease ARE on the rims. When it's raining, water and mud add to the list and braking is no more a speed decrease but just some noise.
Also, when you use your brakes, the particles stuck between rim and pads damage both.
Pad Tip is an apparatus you fix to your brake pads to clean the rim from any particle, liquid or grease.
Slightly closer to the rim than the pad, Pad Tip touches it before to clean the braking surface. The symbolic representation hereby shows the cleaning bit (1) fixed on a pad (2) to clean the beaking surface (4) of the rim (3). During braking, the cleaning bit (1) touches the rim (3) slightly before the pad (2).