In a chemical reaction no atoms are gained or lost, but you rarely collect all the product the equation predicts. The amount of product you actually obtain is called the yield. There are three main reasons why it is lower than the maximum. The reaction may not go to completion because it is reversible. Some product is lost when it is separated from the reaction mixture, for example during filtering or transferring between containers. Some of the reactants may also react in ways different to the expected reaction.
The maximum theoretical mass of product is the mass of product the balanced equation says you should get if all the reactant reacted and nothing was lost. Comparing the actual yield with this as a percentage gives the percentage yield:
% yield = (mass of product actually made ÷ maximum theoretical mass of product) × 100
Example: heating 10 g of calcium carbonate can give a maximum theoretical mass of 5.6 g of calcium oxide. A student actually makes 4.2 g. The percentage yield is 4.2 ÷ 5.6 × 100 = 75%.
Higher tier: you may need to work out the theoretical mass yourself. Use the balanced equation and relative formula masses to find the mass of product that a given mass of reactant should give, then use that value in the percentage yield formula.