Liquid Ratio describes the amount of PG and VG in E-Liquid. Pg and Vg are the Two main ingredients in e-liquid.
Pharma grade Propolene Glycol (PG) which is found in asthma inhalers and food is used to create the throat hit of smoking and to carry flavour
Pharma grade Vegetable Glycerine (VG) which is found in food and the cosmetics industry is used to help create the cloud associated with smoking.
Different mixtures of PG and VG are needed for each devices coils to wick correctly and not leak. Too much PG in a sub ohm kit (DTL) could lead to leaking similarly too much VG in a pen style or low wattage device (MTL) could lead to dry burnt draws.