what is the meaning of foreign key and use

A foreign key (FK) is a column or combination of columns used to establish and enforce a link between the data in two tables. A link is created between two tables by adding the column or columns that hold one table's primary key values to the other table. This column becomes a foreign key in the second table.

You can create a foreign key by defining a FOREIGN KEY constraint when you create or alter a table.

Why we need to use the foreign key in the table?

It ensures that there is an integrity between the tables. For example you have the table called EmployeeMaster. The employee id is the primary key and it is eligible to became the foreign in the child tables. If any body try to enter the salary for unknown employee then its error.

Employee Table:

EmployeeId     Name    DOB         DOJ
1                   S        1/1/1900   5/6/2010
2                   K        5/7/1989   4/4/2011

EmployeeID is PRIMARY KEY, it ensures here it will not allow the duplicate entries and creates the clustered index.

Salary Table:
ID    EmployeeID    Month   Year    Salary
1           1              5        2012    24500
2           2              5        2012    15000
3              3                5          2012    35000 

Here EmployeeID is foreign key, it has the relationship withe Employees table. Here i try to insert the record for Employee ID 3, but it is an not relevant. It throws an error while inserting.