"XDM" stands for "Experience Data Model". But you don't really need to know that for the purpose of a layman's description. Think of it as just a name that Adobe has applied to the schema, e.g. to "brand" it.
A "schema" is a specification of information. In the case of the XDM schema, the specification dictates how data must be arranged into a format (or structure) that Adobe Experience Plartform accepts.
You can think of it like ordering a meal at a restaurant. You are presented with a menu that contains categories like appetizers, mains, desserts and drinks, and you can order as many as each category. But you cannot order something that is outside of it, like groceries, because the chef at the restaurant's kitchen won't know what to do with that. Also, different restaurants can define their own menus, and some may have other categories (e.g. fruits) or omit some categories (e.g. appetizers).
Likewise, the XDM schema is like a restaurant menu. The Adobe organisation is the restaurant, and the XDM schema is its "menu", i.e. the data that the organisation has defined that it accepts. In your implementation ("meal order"), you ("restaurant customer") provide the data ("menu items") according to that schema, but you cannot add anything from a field group ("menu category") that is not included in the schema.
Hope that helps!
