The Win Rate option is calculated as the percentage of paid contests in which the player has finished in the winning zone of the contest. Click on the profile icon of any user and you'll be able to see this info on their Dream11 profile's Career Stats section.
Eg: If your opponent has played 10 paid contests and finished in the winning zone of 5 of these contests, their win rate would be 50%.
PS: This information is currently available only when you visit another Dream11 user's profile. You can view your own paid contest wins on your profile under the 'Playing History' section.