JBOD ("just a bunch of disks," or "just a bunch of drives") is a derogatory term - the official term is "spanning" - used to refer to a computer's hard disks plugged into a controller card that haven't been configured according to the RAID (for "redundant array of independent disks") system to increase fault tolerance and improve data access performance.
Most disk controllers don't have any RAID functionality built into them. In these cases, the Operating System sees the disk drives connected to the controller as Just a Bunch Of hard Drives. In addition, many RAID controllers default to a JBOD configuration before being configured for RAID.
Tim: Hey johnny you can buy a controller card, and set it up as a JBOD array.
Johnny: Wow... I have no idea what you just said, but if you set it up for me I'll let you watch my mom in the shower through a hole in the wall...
Me: "He's just been Jbo'd."