Grams tell you mass; milliliters tell you volume. For liquids and pourable ingredients, density links the two. This page focuses on liquids where converting g→ml is most useful — unlike dry powders where cups are often preferred in US recipes.
How to convert grams to ml
Divide grams by density (g per ml). If you know 100 g of an ingredient equals a known volume from a chart, you are using the same relationship.