Look at the deck of your atomizer. Find the positive post (the post that is connected to the 510 pin on the bottom via screw or post), you can tell which one it is because on most non velocity style decks it is whichever post is center, but if this isn't the case just look at the bottom of the deck and find the post that has a white, plastic looking piece around it. If it's a sub-ohm tank with pre-built heads, it'll be a rubbery/plastic piece that fits between the negative and positive coil leads somewhere on the bottom of the coil.
That plastic piece is the insulator, and it's creating a barrier between the metal on the deck which serves as the negative/ground of the circuit (connecting via the threads of the 510), while the post itself only connects to the 510 pin of the atomizer which then connects to the 510 pin of the mod, which is the positive of the circuit.
This allows the completion of the circuit when firing your mod and allows current to flow through your coils, which are made out of wire that resists the current, causing it to heat up. Hence resistance wire. The amount of current (wattage) flowing through your coils and how much resistance (ohms) those coils have are the variables that give way to how much heat/vapor/etc. your coils produce.
If these two, the negative/ground and the positive, were to touch, it would create a hard short and probably end up venting your batteries and burning up your mod internals, melting your coils, burning your cotton, etc.
A hard short creates what is essentially an infinite loop for current to travel and it WILL do so (and it will be drawing HELLA amperage in the process as this electricity is running freely through EVERYTHING in its path (which is the entire atomizer in this case.)) indefinitely. This will not stop until the short is fixed, the batteries go into thermal runaway from stress (venting) or your disconnect the atomizer / remove the batteries.