Screeps: Arena is an RTS all about programming. Quite literally: You don't control anything once the match starts, you just hope that the orders you planned out beforehand are good enough.
Then your units get thrown into the fight against others in quick 1v1 matches. Those matches are played out in a variety of arenas with various rules, each with its own matchmaking and rankings, and you'll want separate sets of orders for each arena. New arenas are introduced each season of Screeps, and you'll refine your code for those new arenas and refine it again between matches to get better.