A ratio is a way to show the relationship between amounts of something.
In balanced equations, the number in front of each formula shows the ratio of the reactants and products. It tells us how many moles of each chemical we need to react to make the products. If there is no number, the amount is one mole.
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
The molar ratio is:
1 mol Zn : 2 mol HCl : 1 mol ZnCl2 : 1 mol H2