Complex sentence: Although root access is available on the Droid X the bootloader is "locked" and uses PKI to check the signature of the kernel, thus third-party ROMs are limited to using the kernel that Motorola ships with the phone.
Split into simpler sentences: Although root access is available on the Droid X the bootloader is "locked" and uses PKI to check the signature of the kernel. Thus, third-party ROMs are limited to using the kernel that Motorola ships with the phone.