The software that powers your online storefront. The license comes with or without the source code for different levels of control. Source code is not necessary for "look and feel" skin implementation and function customization. Source code is necessary if you plan to own the code and have complete control of how each and every line of code works. The software is licensed per store/domain.
Compare ML/Express and ML Versions. Note that ML/Express version has certain limitations and does not have source code option.