How do I play because I am confused?

I don’t know how to play it is really complicated to me

Take a look at this challenge:

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.