A standard short-fill might be 50 ml of
eliquid with no nicotine in a 60 ml bottle. You're expected to buy it with a 10 ml unflavoured nicotine topup, which you would pour in and mix to add the nicotine. It's a way of getting around restrictions on the size of ejuice bottles that include nicotine.
You can vape the short fill juice straight if you don't want nicotine. However, the flavour might be stronger than you want because it's intended to be diluted a little with the extra 10 ml of unflavoured nicotine. If that's an issue, you can get some plan VG and/or PG and add 10 ml of that instead.
The short-fill is cheaper because it's only part of the juice (if you want to add nicotine) and you have to buy the nic separately.