The theoretical yield is the maximum amount of product you can get from a chemical reaction, based on stoichiometry and assuming everything reacts perfectly. The percent yield tells you how much product you actually obtained compared to the theoretical yield, expressed as a percentage. You calculate it using:
Percent yield = (Actual yield ÷ Theoretical yield) × 100%
For example, if a reaction could theoretically produce 10 grams of salt, but you only collected 8 grams, the percent yield would be:
(8 ÷ 10) × 100% = 80%.
It basically shows how efficient your reaction was.