Yield is the amount of product made during a chemical reaction.
Theoretical yield shows you how much product you should be able gain from the experiment, which depends on how much reactants you put in, the temperature, concentration, pressure etc. However, it is impossible to gain ALL the products as you could lose some of the reactants or products during the experiments. This could be due to evaporation, gas escaping or just some remains of the product left behind in the container, because it is impossible to gain all the reactants as products at the end of an experiment. So it will always be less, than the theoretical yield.