I had some bloodwork done and my albumin was 21. What is albumin, and what does this mean?

Probably lab error. Albumin is a protein made by the liver and is measured through blood testing. It is usually used as a marker of nutrition or liver disease. Normal values are generally between 3.5 to 5. The value of 21 is unusually high and is most likely an error.
Blood protein. Albumin is a protein, produced by the liver, that is found in the blood. It does a lot of stuff including carrying drugs and other stuff around as well as maintaining the correct osmotic pressure in your blood. As a blood test, it is a measure of liver function. The doc who ordered the blood tests should explain the results to you at your next encounter.