ARP stands for Address Resolution Protocol which basically refers to the resolution of an Address (Your IP address within a switched network) to a MAC (Media Access Control).
The idea of ARP Cache Poisoning is that you "inject" a IP to MAC mapping inside of a targets ARP Cache. Lets pretend that these ARP Cache Tables work sort of like DNS tables for ease of understanding. Basically there is a cache kept of IP to MAC mappings for the switch, and by putting in a new one (usually one that points to your own machine inside the network) you can intercept traffic for that machine.
This type of attack is also commonly know as the "man in the middle" attack. The basic idea as I see it is to poison the arp cache forcing traffic that is supposed to go to and from your target machine to actually going to you. You could then setup ipforwarding (or something like it) to continue the communication out to the internet, and back to the target, thereby making you a stop in the middle where you could sniff all incoming traffic just as easily as if you were all on a hub.
Short of this, you could also ARP flood the switch. By putting to much stress on a switch with this ARP flooding, you could force the switch to go into a "safety" mode where it will act as a hub. Then you could sniff traffic again like you were on a hub and not a switch.
As I said, I don't know if that is what this question was going for or not, to vague.
