Why scientists care
Stoichiometry turns a balanced chemical equation into a quantitative relationship between reacting species.
The coefficients are ratios of entities or amounts of substance; they are the bridge from a symbolic reaction to laboratory quantities.
A concrete example
For 2 H2 + O2 → 2 H2O, two moles of hydrogen react with one mole of oxygen in the balanced ideal ratio.
Try to notice
Science becomes memorable when a definition is connected to something measurable or observable.