Short fill e-liquids are zero-nicotine liquids supplied in “short fill” bottles: these are large bottles which have been left with a bit of empty space. This means you can pour a nicotine shot in to give yourself a larger bottle of e-liquid, now with nicotine! Give them a quick shake to make sure the nicotine is mixed in (hence the alternative name, "shake 'n' vape") and they're ready to vape.
60ml bottles of KonceptXIX, for example, contain 50ml of 0mg e-liquid, leaving just enough space to add a 10ml nicotine shot. This gives you 60ml of newly nic'd up e-liquid.
Short fill e-liquids are a popular choice as they’re great value for money, and let you decide exactly how much nicotine you want your liquid to contain. You can find short fill juices in a wide range of flavours and from all sorts of popular brands, so you're sure to find something for you!