A short fill is a larger than 10ml bottle of 0mg flavoured e-liquid that has been over flavoured and under filled to allow room for a nicotine shot. You may ask why wouldn’t you just buy a big bottle with nicotine in. There are laws in Europe that prohibit the sale of nicotine containing bottles over 10ml in capacity This is called the TPD or Tobacco Products Directive. The TPD does not cover 0mg or Nicotine free liquid so with short fills you can buy a big bottle of liquid and add nicotine after the fact. There are limits though as often the highest you can achieve is 6mg and usually only in a higher VG content.
If you use a mouth to lung device or your coils struggle to keep up with 70 Vg, a High PG shot is recommended to thin the liquid enough to allow you to use a bigger bottle.