What does bad ESN mean?

I'm not very good with technology so this might be a stupid question but if a cell phone has bad ESN does that mean it doesn't work? I'm looking at a Motorola Droid 2 on Ebay and the item description says "bad ESN". I'm not sure what that means I just want to make sure I don't bid on a defective phone without knowing. Thanks for any help in advance!