In DBMS you may hear cardinality term at two different places and it has two different meanings as well.
In Context of Data Models:
In terms of data modeling, cardinality refers to the relationship between two tables. They can be of four types:
One to One – A single row of table 1 associates with single row of table 2
One to Many – A single row of table 1 associates with more than one rows of table 2
Many to One – Many rows of table 1 associate with a single row of table 2
Many to Many – Many rows of table 1 associate with many rows of table 2
In Context of Query Optimization:
In terms of query, the cardinality refers to the uniqueness of a column in a table. The column with all unique values would be having the high cardinality and the column with all duplicate values would be having the low cardinality. These cardinality scores helps in query optimization.