Foreign Keys are basically the glue that adheres database table together. A Foreign Key is a field in one table that refers to a Primary Key in another table, allowing them to be connected. For example, if we have a cars list in table "Cars", the Mercedes with ID "2" is owned by employee "5" (Foreign Key), and John Doe happens to have ID "5" (Primary Key) in "Employees" table. Now we can say that John Doe owns the Mercedes.
This connection gives us the ability to join 2 or more tables into 1 table, which gives us more flexibility in creating meaningful charts.