How to use RestAPI(cURL)... Function of "LocalLB::Rule::get_description"

Not answering your question directly, but "LocalLB::Rule::get_description" is part of the original iControl API (see Wiki: iControl : API Reference); the newer iControl REST API is different (see iControl REST Home). What specifically are you trying to achieve?

You may want to consider using the Python SDK. In that case, 'description' is a property of any object that has one. For example:

!/usr/bin/env python3

from f5.bigip import ManagementRoot
from getpass import getpass
from pprint import pprint

hostname = 'my.f5.ltm.net'
username = 'foo'

mgmt = ManagmentRoot(hostname, username, getpass())

virtual = mgmt.tm.ltm.virtuals.virtual.load(
    partition='Common',
    name='my_virtual_server',
)

if hasattr(virtual, 'description'):
    print(virtual.description)
else:
    print('Virtual server does not have a description.')