Hi Priyaranjan,
Seems you are new to Smarform Devps, input and output paramters for SF(s) are the parameters which are used when calling the Smartform from the print program, as you may come to know in future or is known, that the Smartforms have its existence in SAP in the form of Function Modules, hence the input and output parameters in Smartform are same as import and export parameters in Function Module.
When it come to Program Lines, it has more to do when you want to include some set of code in Smartform at Node Level, then you can do with Pragram Lines.
Please check at your edn and let me know in case of further inputs.
BR/Thanks
Pranav Agrawal