Often this term stands for modification; in reference to vaping pens, a mod is simply a vaping pen (type of electronic cigarette) with a bigger battery, and more functionality to give a device which holds a charge longer, produces more flavor, more "throat hit", and usually offers other functions such as variable voltage, variable resistance, and recently temperature control. Mods are continually being improved, e.g. with computerized electronics which can be upgraded via bluetooth connection.
Have you seen the newest mod by Inokin? It has overcharge protection, a puff counter, temperature control, and allows for sub-ohm vaping for more flavor.