Can anyone explain me what is the ARP Spoofing or ARP poisoning?

Ethernet ports don't have up addresses, they have a Mac address which is globally unique and assigned at the factory. When a computer wants to talk on a lan, it has an ip address and uses Arp to look up the mac, at a hardware level the packet is sent to the Mac address; Ethernet switches only know the Mac address.

By tampering with Apr you can make network traffic go the wrong way which sets the stage for all kinds of abuse.