A kick is a device that fits in the battery compartment on the positive end of the battery. It converts the device to a variable wattage device. Some devices like the Legacy, Silver Bullet, Siam Cube, and Bolt can extend to take a kick with an 18650. However, you can use it in any device that will take an 18650 by using an 18490 or 18500 battery along with it.
Since you don't know what a kick is, you probably don't know what variable wattage is. Watts are the energy used by the device. The Kick will detect the resistance of what you put on the device and adjust the voltage to keep the watts constant. In theory that means you get the same experience no matter what you put on it. It can boost the voltage beyond what the battery provides.
My own experience says that it works great to adjust to the changes in an atomizer as it wears. So, once you get it adjusted, it works great as long as you don't change to a different type of atty. However, if you go from say a 2.5 ohm 901 to a 1.7 ohm 306, you'll probably find you need some adjustment and it's not that easy to adjust. It also goes through batteries pretty quickly.