Let's examine this vulnerability from both "definition" and "impact" sides.
iSCSI is a protocol (i.e., a way to interface / talk with) associated with SAN disks that are part of a Storage Area Network. What it means is, you can interact with the SAN disks using tools that implement this protocol (e.g., iscsiadm can be used to talk to a SAN disk).
What this vulnerability says is this - SAN disks in this network are accessible and they won't even ask you the password! Feel free to browse through what we have kept in our huge storage network (for which we paid, and are continuing to pay, a big sum because the data is important to us)!
This article demonstrates how penetrations testers became domain admin through this vulnerability.
This paper, presented in BlackHat, provides further information about various weaknesses in iSCSI.
Hope it helps.