The Photography Challenges are a collection of projects which you can use to learn and improve your photography.
Start each challenge by reading the explanatory text and watching the video. Then examine the top photos from the project library for inspiration.
Next compare pairs of photos from the challenge library, deciding which one is best and writing a short explanation of why you think it's a good photo in terms of that particular challenge. This helps you learn the concept more deeply.
Finally submit your own photo to the library. Once it's approved you'll have unlocked the next project.
While a fun part of the challenges, the ranking should not be taken too seriously. Most people take part in the challenges to learn and have fun rather than compete for a high score.
As part of completing a challenge each person compares a set of previously submitted photos.
These comparisons help the learning process, however they are also used to rank the photos in each project using the Elo Rating System which you can Google if you're interested in the technical details.
Each photo starts with a rough score given by the editor who approved the photo to begin with. Then as it wins or loses comparisons it gains or loses a number of points based on the existing points (rank) of the photo it was compared with.
Each comparison is randomly assigned. If a photo wins a comparison against a high scoring photo it will gain more points than if it won against a low scoring photo.
A photographer's total number of points is the sum of the best photos they have submitted to each challenge. So it is possible to improve your total score and rank by submitting better photos to each project than those you've already submitted.