I don’t know how to play it is really complicated to me
Take a look at this challenge: https://www.codingame.com/training/easy/onboarding
It describes what to do and how you can achieve it.
In general: you have to code to solve different tasks. In the training section you compete against a given test-set, in multiplayer your code is compared against the code of other players and if your is better you rank better.
For the beginning I suggest you do the easy tasks on training and move on to multiplayer to see how it works. And contests are timed multiplayer games.