Actual Yield is the mass of the desired product in the actual experiment.
Theoretical Yield is the mass of desired product if everything went perfect in the experiment. This is usually calculated from book knowledge or from experimental data.
Example
If an experiment yielded 21.7g Cu, but was supposed to yield 22.0g Cu, find the Percent Yield.
Answer: (21.7g / 22.0g) x 100% = 98.63% = 98.6%
Percent Yield Problem Set