A Composite primary key is a set of columns whose values uniquely identify every row in a table. What it means is that, table which contains composite primary key will be indexed based on columns specified in the primary key. This key will be referred in Foreign Key tables.
For example – if combined effect of columns, “Employee_ID” and “Employee Name” in a table is required to uniquely identifies a row, its called a Composite Primary Key. In this case, both the columns will be represented as primary key.