So, you were thinking about picking up a Kindle Fire HD, rooting it, and throwing a ROM on it for an impressive $200 tablet? Turns out that idea may not work out as well as we initially thought: both the Kindle Fire HD and the second gen KF have locked bootloaders. Bummer.
This may not mean that custom ROMs are impossible on these devices, only that it's more improbable.
For those who may not know, the bootloader is responsible for checking the firmware's signature before a device boots. In this case, if it doesn't match what Amazon says it should, then it simply won't do anything.