In simple terms, the bootloader is the utility that allows custom ROM builder to load replacement software into the firmware. This involves taking superuser status, or "rooting" the device, that allows system software to be replaced. The modding community is reporting that Motorola's new Atrix 4G has a locked bootloader that is designed to prevent rooting and changing the firmware.
This is not the first Android device to be locked down, and while it complicates the modding process the developers behind it are resourceful and will likely get past the locked bootloader in time. Locking it down is obviously Motorola's way of making it much more difficult to keep casual modders at bay.
While Motorola tends to take securing the system software on its devices seriously, it appears the upcoming XOOM tablet will not be locked down. The company began tweeting yesterday that the XOOM bootloader will be "unlockable/relockable to enable developers to access hardware for development".