The mod carries batteries and the needed electronics for firing your atomizer.
There are basicly 2 kind of mods: mechanical mods and regulated mods.
Mechanical Mods work like a Flashlight. If you press the button the circuit with your heating wire is closed and your vape fires.
Regulated mods have advanced electronics that provide various protections and safeguards.
They also provide Features like Variable Wattage to configure the Power that is consumed by your atomizer.