An entity relationship model ("ER Model") is a simplified description of what "entities" exist (see: Entity Definition) and how they are related to one-another (Entity Relationship Categories). They are useful in Software & Systems Engineering, in business, and potentially useful in any other domain of study. Interestingly, there isn’t really a UML ‘Entity Relationship’ diagram - but they use Class Diagrams, which are similar..
Entity relationship models name independent entities and describe how they relate to one-another via relationships, which themselves can be named, that may or may not be subject to constraints. Common relationship constraints include "cardinality" and also "role/type acceptance".
There are a number of competing Entity-Relationship Modeling Standards. I tend to like either Crow’s Feet Notation or the even simpler yet more descriptive Min-Max pairing.