Nic Shots allow E-Cigarette users to add nicotine to their 0mg Strength E-Liquids. Usually Nic Shots are flavourless and only increase the overall nicotine content of the bottle they're being added to. An example of this can be found below.
In other cases, nicotine shots are salt based - where nic salts allow for a quicker intake of nicotine into the body.They also reportedly improve flavour, although this is subjective.
Nic Shots can also come with a 'Flavour Boost' - a single note diluted concentrate that increases or adds the particular flavour of the Nic Shot. The most common example is a 'Cool Shot' which contains koolada (WS-23 for the nerds) - this makes the host E-Liquid 'colder' as a result (cold being similar but distinct from menthol).