Superman is an alias of Clerk Kent
Clerk Kent is an alias of Christopher Reeve
We reference same person with a different name.

A Table or a Field can be referenced with an alias (another name)

SELECT CustomerName AS Customer, ContactName AS Contact FROM Customers;

Another use of Alias

SELECT o.OrderID, o.OrderDate, c.CustomerName
FROM Customers AS c, Orders AS o
WHERE c.CustomerName="HardCode Systems" AND c.CustomerID=o.CustomerID;

Above SQL statemen can be written without Alias 

SELECT Orders.OrderID, Orders.OrderDate, Customers.CustomerName
FROM Customers, Orders
WHERE Customers.CustomerName="Hardcode System" AND Customers.CustomerID=Orders.CustomerID;

Alias are useful in JOIN queries