It's a battery mod that can be personalized.
The skinny little batteries that come with e-cigarettes don't last very long between charges. Once you have been vaping for a little while, you will begin to get annoyed at how frequently you have to take the battery off, put it in the charger and put a fresh one on.
So, clever people like Jeff, who makes The Chuck, have developed battery modifications that allow you to use large, rechargeable batteries. You can go a day or even longer without changing batteries.
Some, like the big Chuck (but not the Little Chuck) can also use a combination of batteries that let you vape at higher than the standard 3.7 volts. This changes the volume of vapor, the taste and the throat hit. Many people prefer it to 3.7 volt vaping.