The most simple form of this would be taking an unregulated mod and installing a device called a kick into the tube to regulate the output of the battery. Another simple solution to attain the same result would be by using a box mod which comes with a potentiometer installed to vary the voltage such as in the Hex Ohm mod.
More complicated regulated mods offer additional features such as: variable voltage, variable wattage, temperature control, memory settings, puff counters, ohm meters, average amp draw readings, PBC temperature, and more. They also offer the possibility of adding in safety features such as: reverse polarity protection, short circuit and/or open circuit cut offs, low ohm cut offs, and many more. It’s important to note that while a regulated mod may offer many safety features, they are intended to be safety nets in emergencies and should supplement your own safety measures (such as taking care of your gear and batteries and educating yourself) and never replace your own safety measures or be relied upon solely.