The weight class it's about the relationship between your channel size and the keywords in your weight class based on their competition score. It's something we compare to the amount of competition on a tag to show your channel's percentile on YouTube, so you can have an idea about your channel's rank in overall YouTube population. But, the actual utility of it is showing the tags that can help your channel the most based on their competition score. If your weight class is higher than the competition, videos on your channel are more likely to show up in search results for that tag. If it’s lower than the competition, videos are less likely to show up in search results for that tag. Here what the colors mean:
Green means "your weight class is above the competition"
Yellow means "your weight class is within the competition"
Red means "your weight class is below the competition"
For more information, you can contact firstname.lastname@example.org