Mouth to Lung (MTL / ML) means inhaling by filling the mouth with vapor before further inhaling into the lung.
It is essentially the same as how most people drag on a cigarette. You suck the vape into your mouth and then inhale into your throat or lungs. People that MtL aren't focused on generating huge clouds of vapor. Instead, flavor and nicotine are key to delivering the satisfaction they crave.
Similar to the way you puff a cigarette
Think about drinking through a straw instead of chugging a beer