Domain Modeling is a way to describe and model real world entities and the relationships between them, which collectively describe the problem domain space.
Entity Relationship Diagrams (ERD) are used widely in domain modeling. In this diagram, entities are showing by boxes and are connected to each other with a line (relationships).
An example of ER diagrams are shown below:
To really learn about this topic, please read this Wiki page
To have a rough overview, you can watch this video {% hyf-youtube src="https://www.youtube.com/watch?v=-fQ-bRllhXc" %}