Hello, rim grinding is where you roughen the braking surface on your rim, resulting in more friction between your pads and your rim, therefore making your brake have better hold. you can do this by grinding the braking surface lightly with a grinder, hold it at a angle so that u dont cut your rim in half. Also keep moving the rim or the grinder so you dont end up with an uneven grind and unlevel braking walls as this might result in your rim being weaker or flexing more. hope ive helped chris.