A chemical reaction often makes more than one product, and usually only one of them is the one you want. That one is the desired product. The rest is waste, unless someone can find a use for it. Atom economy tells you what percentage of the mass of the reactants ends up as the desired product.
To calculate it, take the relative formula mass of the desired product from the balanced equation, divide it by the sum of the relative formula masses of all the reactants, and multiply by 100. The total mass of the reactants equals the total mass of the products, so the two ways of describing the total give the same answer.
A high atom economy means most of the starting material is turned into something useful. This is better for the environment because less waste is made and fewer raw materials are used up. A reaction with only one product, such as an addition reaction, has an atom economy of 100%. A reaction that makes a lot of waste has a low atom economy.