How is FOREIGN KEY commands related to the PRIMARY KEY?

Q. How is FOREIGN KEY commands related to the PRIMARY KEY?

Answer =

A primary key-foreign key relationship defines a one-to-many relationship between two tables in a relational database. A foreign key is a column or a set of columns in one table that references the primary key columns in another table. The primary key is defined as a column (or set of columns) where each value is unique and identifies a single row of the table.