In a relationship between two entities, cardinality captures how many of one entity (entity instances) participates in the relationship with how many of the other entity. Cardinality is represented by the symbols that appear on both ends of a relationship line. Data rules are specified and enforced through cardinality. Without cardinality, the most one can say about a relationship is that two entities are connected in some way.
For cardinality, the choices are simple: zero, one or many. Each side of a relationship can have any combination of zero, one or many ('many' means could be more than 'one'). Specifying zero or one allows us to capture whether or not an entity instance is required in a relationship. Specifying one or many allows us to capture how many of a particular instance participates in a given relationship.
These cardinality symbols are illustrated in the following information engineering example of Student and Course.
The business rules are:
Each Student may attend one or many Courses.
Each Course may be attended by one or many Students.
DAMA Data Management Body of Knowledge 2nd Edition, 2017, Print.