There are 3 main components to e-cigs. There's a battery, an atomizer and e-liquid.
The atomizer is essentially a coil of wire and a wick. The e-liquid contains some mixture of flavouring, propylene glycol and/or vegetable glycerin (PG/VG) and may also contain some concentration of nicotine. This e-liquid saturates the wick. The battery sends current through the coil which vapourizes the e-liquid which is then inhaled by the user.
What you're looking at in that link is a fancy battery holder that allows the user to change the voltage delivered to the coil. It also measures the resistance of the coil so the user can specify a desired wattage and it's "smart" enough to deliver the correct voltage to achieve that wattage.
This video is 3 years old, so some of the tech that he talks about is obsolete now (like carto tanks), but the concepts are still the same.
A PBusardo Video - Beginners Guide to E-Cigs and E-Cig Tech
https://youtu.be/iVFSk5fW7bo