Diep io is a game where you have to attack enemies a lot. When the game was launched you had to hold LMB in order to maintain highest reload speed and fire rate. It was very tough and developers decided to implement auto attack feature. This feature allows player to keep shooting without holding LMB.
Auto fire mode can be switched on by pressing E keyboard key. You can switch this mode off by pressing this key again.
When this mode is active your tank will always be shooting with maximum reload speed. This mode is great for bursting.