A domain model is a data model containing the entities of a certain domain. A domain is a functional restriction for the module that it belongs to.
Check out the Mendix Academy learning path for data modeling: https://gettingstarted.mendixcloud.com/link/path/59/Expand-Your-Domain-Modeling-Skills
Additionally to Tim; Based on the domain model the Database structure with tables and columns is created. Though the database doesn’t equal 100% the domain model. Calculated attributes and Non-persistent entities (the orange entities) are not part of the database structure. Also the ID’s use for references (foreign keys) are not visually present in the domain model, while there is a (by Mendix maintained) ID for each record in the database and part of the objects in runtime. These ID’s are used to set & track the associations in join tables, which are reflecting the associations you draw in the domain model.