This is the liquid used in vaporettes/electronic cigarettes to make a smoke-like vapor.
Propylene glycol and/or glycerine are the ingredients that give the vapor body like smoke. Flavourings and alcohol are also usually included. Nicotine is added in different strengths from zero to 36mg per 1ml.
I drip a couple of drops of e-liquid on my atomiser to make vapor. It is similar to smoking but it tastes better.