This is used to get the value of that parameter which is used in client script

for example

in client script it should be written as

ga.addParam('sysparm_sysid',g_form.getUniqueValue());

here we pass a value with this parameter and in script include it should be written as this.getParameter('sysparm_sysid') to get value of it.

Here sysparm should be mandatory but we can add name with it as i shown above