The theoretical yield assumes every reactant turns into product and every bit of product is collected. In real experiments the actual yield is almost always less than the theoretical yield, so the percentage yield is below 100%. A yield of 100% would be rare. For example, an 82% yield means 18% of the possible product was not collected, and a 65% yield means 35% was not collected.
One cause is an incomplete reaction. Some reactions are reversible, so the products turn back into reactants. In other cases the reaction stops before every particle has reacted. Either way, some reactant is left over and less product forms than the equation predicts.
Another cause is practical losses during the experiment. Product can be left on the filter paper, stuck to the sides of a beaker, or spilled while transferring a solution. Some may evaporate, or stay dissolved in the liquid you filter off. A third cause is a side reaction, where the reactants make an unwanted by-product instead of the desired product.