The commercial license allows you to provide commercial software licenses to your customers. Basically if you are licensing the software you develop using MySQL as open source you can use it for free, but if you plan on keeping the source code yourself then you have to pay the commercial license fee. According to MySQL's website "Commercially licensed customers get commercially supported product with assurances from MySQL." The good thing about this licensing structure is that you don't have to buy the commercial license until you try to distribute software that is non GPL compliant.