In C#, method that has the same name as the class is called constructor. It serves as...well...constructor of the class instance (when you call new). Any parameters passsed into it are available at the moment of class creation and serve for initialization of the instance state.
Example would be...hm...you double click on the grid containing list of orders from a client. Double click opens new form and passes in orderID like this
frmOrderDetails items = new frmOrderDetails (dgvOrder.CurrentRow["order_id"])
Then you can in details form save that ID and recover from the database everything you need to fill that particular order details.
If this helps please take time to accept the solution. Thank you.