Items are measured in inches flat. For example, if an item says "bust: 18," that means if you are looking at the item straight on, the measurement from armpit to armpit is 18 inches. This measurement needs to be doubled for the circumference, which would be 36 inches.
XS: B 16-22 W 13.5-19.5 H 25 L 46
translated means:
Extra small bust is 16 inches flat unstretched, and stretches up to 22 inches flat. The circumference would stretch from 32-44 inches. The waist is 13.5 inches flat unstretched, and stretches up to 19.5 inches flat. The waist circumference would be 27-39 inches. The hip measurement is 25 inches flat or 50 circumference. The length is 46 inches from shoulder to hem.
Other words you might see are "rise" and "inseam." Rise is the measurement from the waistband to the bottom of rate crotch, and inseam is the measurement from the crotch seam to the hem of pants.