I will just add small 'how to' for starters.
Open terminal and type (if you want to save to Desktop, go to your Desktop path with cd Desktop command):
nano arp-poisoner.py
Copy and past script code from 0x0ptim0us post. (To save new file with nano editor, CTRL+X, type Y, then Enter)
Set permission for file :
chmod +x arp-poisoner.py
To run script:
./arp-poisoner.py
I like simple scripts. Thanks for share.