In diep.io you play as a tank and try to shoot down your opponents and level up as much as possible.
There are also little shapes on the map that you can destroy to gain XP levels. With these levels you will be prompted to choose an upgrade for your tank like health, damage, speed, reload time, etc. This opens up the opportunity for many different strategic builds to be made. On top of the upgrade system, you also get to evolve into different types of tanks at certain levels. I will tell you that this game is an insane amount of fun. Give it a try and see how far you can get. The max level is 45 at the time of writing this, so see if you can reach the cap and dominate the server!