An example is provided below to make it more understandable:
Weight Max MinutePower (MMP) W/KG Ratio
Cyclist One 70 kg 350 5
Cyclist Two 90 kg 400 4.4
You might expect cyclist two, with the higher Maximum Minute Power (MMP) score, to continually be ahead of cyclist one. There are, however, a number of situations where higher power does not give you the advantage. This could be climbing up a long, steep hill or during a short distance sprint. In these instances, where gravity and drag have an influence, the cyclist with the higher power to weight ratio will often come out on top. 
Why would I need to know what my Power to weight ratio is?
Working out your power to weight ratio will help you compare your ability to your friends and fellow cyclists, no matter what size you are.
What is a typical power to weight ratio?
Examples of typical power to weight ratios can be seen in the table below which is based on a 3 min max test:
Rider Type W/KG based on three minute test
Power to weight ratio in our view is an essential statistic to obtain if you are looking to improve your cycling and overall fitness levels. Creating a training program that adjusts weight in a scientific and effective way (reduce body fat and retain/add muscle) is a essential. Imagine what could happen to your power weight ratio if you lost the unwanted fat your retain and replace it for lean, powerful muscle tissue. Your power potential would increase through training and your overall weight would decrease therefore increasing your power to weight ratio and therefore making you a more likely winner!
The Soho Fitness Lab Team
 Watt Bike UK (https://wattbike.com/uk)