Glass Mapper is an open source library which facilitates mapping Sitecore item data into your domain model. Active Commerce utilizes Glass Mapper for populating domain object data for products, orders, configuration, on-page content items, and other entities. If you are extending an Active Commerce class which utilizes Glass Mapper, you review the tutorials provided on the glass.lu website for Glass.Mapper.Sc.
Glass Mapper does require knowledge of any assemblies which include mapped classes. Active Commerce makes this easier by providing a configuration element where you can specify the assemblies which should be registered with Glass. For an example of configuring your assembly, see the Custom Product example in our code samples.