Atom economy is a measure of how much of the starting materials ends up as the desired product. It is the percentage of the total mass of the reactants that becomes the useful product you want. The rest of the mass ends up in unwanted by-products.
A reaction with a high atom economy turns most of the mass of the reactants into the desired product, so it makes little waste. This is better for the environment, and it uses raw materials efficiently, which can lower costs. A reaction with a low atom economy makes a lot of waste.
If a reaction makes only one product, then all of the atoms in the reactants end up in the desired product and the atom economy is 100%. This is true of addition reactions that join two substances into one. Atom economy depends on the balanced equation, and it is a different idea from percentage yield, which measures how much product is actually collected.