Power to weight ratio, racing weight, watts per kilograms or pounds. No matter how you word it the meaning is pretty simple; you need the strength to move that weight. The less you weigh and the more power you generate and the faster you'll propel that bike. Generally speaking heavier riders generate the more watts or power. If your specialty is time trialing or sprinting that would be considered fine but when climbing the situation is totally turned. The higher your body weight the more power you need to fight gravity. To give you an example if I was riding uphill at 250 watts against a rider that was 25 - 30 pounds less that rider would only need approx 215 watts to be at the same speed. The steeper the grade the more this becomes a factor. Now if I could lose 25 pounds but keep my strength I'd certainly crank uphill. Of course losing weight and keeping your power is key because some muscle loss is going to occur with most weight loss and affect my overall power affecting time trialing and sprinting performance.
Bradley Wiggins is a rider who totally amazed me. Watching him years ago he was a spectacular track rider and time trialer but not much of a climber. With an incredible nutritional and training program he lost weight became a first class climber won the Tour de France and still broke records time trialing and rode Paris Roubaix to a top 10 placing showing he maintained his overall power or watts produced. Incredible feat indeed.