Ratan,
Kindly refer Amibroker User Guide whenever you have doubts in functions. All the functions are beautifully explained with clear examples there. If you aren't able to understand them, you can post here, and we can help you out.
Meanwhile, Param() function allows the user to specify the parameters for the functions. In this case, the Lookback period is made configurable. ie at runtime you can change the parameter from Parameters dialog box (Ctrl + R).
If you see the signature of Param(), the second parameter is the default value. You can change it to what ever value, you want, say to 5 in two different ways:
1. Press Ctrl + R, and change it (this is the best method)
2. Change in the AFL code.
Praveen.