what does white list method does? why it is declared first line or above any method?
If you declare any method as whitelisted, it becomes available to call from client. Then you can call that function from javascript or using rest api. You need to login to call those method.
we pass parameters allow_guest= True is there any other parameters avilable like this?
I think there is no other arguments like this. To know more about rest api, check https://frappe.io/docs/user-guide/rest_api