Stoichiometry predicts the maximum product amount allowed by the supplied reactants. This predicted amount is the theoretical yield.

The amount of product actually obtained in an experiment or process is the actual yield. It can be smaller because the reaction is incomplete, competing reactions form other products, material is lost during separation, or the product is not recovered perfectly.

The percent yield compares the actual yield with the theoretical yield:

$$%\text{ yield}=\frac{\text{actual yield}}{\text{theoretical yield}}\times100%.$$

Suppose stoichiometry and the limiting reactant predict $12.5,\mathrm g$ of product, but purification recovers only $10.8,\mathrm g$. Then

$$%\text{ yield}=\frac{10.8}{12.5}\times100%\approx86.4%.$$

The actual and theoretical yields must refer to the same product and be expressed in compatible amount or mass units before taking the ratio.

A measured yield above $100%$ does not mean the reaction created extra matter. It usually signals that the recovered sample contains solvent, impurities or another measurement/model error, or that the assumed reaction description is incomplete.

Yield separates two questions that stoichiometry alone cannot: how much product could form ideally? and how much usable product was actually obtained?