amount of substance
When you buy eggs you ask for a number — a dozen, two dozen — not a weight, because what matters for a recipe is how many. Chemistry works the same way: reactions combine particles in whole-number ratios, so what really counts is how many particles you have. The amount of substance is exactly this count, expressed in the chemist's unit, the mole.
Formally, amount of substance, usually written n, is a base physical quantity measuring the number of elementary entities in a sample, with the mole as its SI unit. It is not the same as mass and not the same as volume; it is a head-count of particles. You convert to it by dividing mass by the substance's molar mass, and from it you can find the actual particle number using the Avogadro constant.
Amount of substance matters because balanced equations and stoichiometry are written in its terms: two moles of hydrogen meet one mole of oxygen to make two moles of water. Working in amount, rather than mass, keeps those neat whole-number ratios in plain view. The one caution is always to state what entity you are counting — atoms, molecules, or ions — since the count differs for each.
Twelve grams of carbon and one gram of hydrogen weigh very differently, yet each is one mole — the same amount of substance, the same head-count of about 6 × 10²³ atoms.
Amount of substance counts particles, not grams — equal amounts need not weigh the same.
The everyday phrase amount of a chemical often means its mass, but in physical chemistry amount of substance is a precise technical term meaning a count in moles. Keep mass and amount firmly apart.