FKs are used most often to ensure that when a row in one table is updated that related rows in another table are also updated.
Example
Table: Employees
EmployeeID - Primary Key
Table: Timesheets
TimesheetID - Primary Key
EmployeeID - The employee for this timesheet
If you want all employee timesheets to be deleted when the employee record is deleted you would go to the Timesheets table and add a Foreign Key.
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This will then result in a Foreign Key on the Timesheet table that will enforce that all rows track what happens to Employee records.
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The way to read this information is that the Primary Table Employees.EmployeeID Column will check with the Timesheets table whenever an update or delete is performed. In this case both operations will result in the same operation for matching entries in the Timesheets table.
If an EmployeeID is changed the Timesheet entries for that column will also be updated.
If an EmployeeID is deleted the Timesheet entries will be removed as well.