A balanced symbol equation can be read in terms of moles. The equation Mg + 2HCl → MgCl2 + H2 shows that one mole of magnesium reacts with two moles of hydrochloric acid to produce one mole of magnesium chloride and one mole of hydrogen gas. The balancing numbers give the ratio of moles.
This lets you calculate the mass of a reactant or product from a given mass. First convert the known mass to moles (moles = mass ÷ Mr). Then use the balancing numbers to find the moles of the substance you want. Finally convert to a mass (mass = moles × Mr).
Example: 12 g of magnesium (Ar = 24) is 0.5 mol. It reacts with 1 mol of hydrochloric acid and makes 0.5 mol of hydrogen and 0.5 mol of magnesium chloride. Magnesium chloride has an Mr of 95 (Mg 24 plus two Cl at 35.5), so 0.5 mol has a mass of 47.5 g. Hydrogen, H2, has an Mr of 2. Because the mass of one mole is the Mr in grams, 24 g of magnesium is 1 mol and makes 95 g of magnesium chloride.