This show what percentage of PG ( Propylene Glycol ) present compare to the percentage of VG ( Vegetable Glycerin ). So a 30/70 PG/VG ratio in an E-Liquid means in 100% pg/vg composition 30% is PG and 70% is VG. Higher the VG percentage denser the vapour/clout and thicker the liquid. Higher the PG value greater the throat hit.