what is foreign key in database with example

Example 1: What is foreign key?

Foreign Key is a non-key attribute which is derived from the primary key 
of another table which links those tables together.

Example 2: what is foreign key

Foreign Key: 
It is a column that comes from a different table and
using Foreign key tables are related each other
It is the primary key of another table
It can be duplicate or null for another table