To understand what is fencing device, you need first know something about split-brian condition. read here for info: http://linux-ha.org/wiki/Split_Brain
Here's is something about what fence device is:
Fencing is the disconnection of a node from shared storage. Fencing cuts off I/O from shared storage, thus ensuring data integrity. A fence device is a hardware device that can be used to cut a node off from shared storage. This can be accomplished in a variety of ways: powering off the node via a remote power switch, disabling a Fibre Channel switch port, or revoking a host's SCSI 3 reservations. A fence agent is a software program that connects to a fence device in order to ask the fence device to cut off access to a node's shared storage (via powering off the node or removing access to the shared storage by other means).
To check whether a LUN has SCSI-3 Persistent Reservation, run the following:
root@doxer# symdev -sid 369 show 2040|grep SCSI
SCSI-3 Persistent Reserve: Enabled
And here's an article about I/O fencing using SCSI-3 Persistent Reservations in the configuration of SF Oracle RAC: http://sfdoccentral.symantec.com/sf/5.0/solaris64/html/sf_rac_install/sfrac_intro13.html