Template describes the structure and behavior of your game object (item, monster, construction,...). As a programmer, you can think of it as a Class. A template has to have a unique name and may contain a set of parameters.
Parameter describes a part of a Document, storing some value. Each parameter has a name and a type. Type can be simple like string, int, float, bool, or a reference to any other Template. As a programmer, you can think of a parameter as a Field or Property of the Class.
Document is a unique instance of a Template (Specific Item: Hunter's Bow, Gold Bar,...), which has its parameter values. Think of it as an instance of a Class as a programmer.
Singleton is similar to the Document, but only one instance of such Documents can be selected and will be used in the app.
Component is a simple Template, which doesn't have its Documents and can exist only inside of another Document.
Enum is an enumerated type, which you can create and use in Data Editor.