Constructors are used to create new objects.
Think of objects as data bags. Typically you would create a new object, and then manipulate the data inside that object.
Constructors are used to create instances of your classes and at the same time initialize the variables. but it is also possible to call a constructor without any parameters. A class can have many constructors and one constructor can even call another constructor. Read the above link or any decent book on Java and you'll understand the concept clearly.