Some bootloaders are locked by the manufactures to prevent the phones from booting unofficial firmware images.
As the excerpt already indicates, a "bootloader lock" is set by the manufacturer to keep users from "tampering too much" with the device. So with a locked bootloader, you usually can not install a custom-rom or even a custom recovery.
Implementations of bootloaders differ, so "hacks" to unlock them are often pretty device-specific (and sometimes even only work with a specific stock-rom version). Some devices are even not un-lockable, on others this step is not reversible (as they e.g., use "fuses" which you need to "burn", e.g. the lg-optimus-4x), and others permit you to switch between "lock" and "unlock".
A common method working on most devices is using adb to run adb reboot oem-unlock. For security reasons, this will include a factory-reset (e.g. to prevent phone thieves from getting access to your data).
More details can be obtained, for example, from the following questions and their answers:
- What is meant by locked bootloader?
- what is a bootloader unlocked phone?
- What's the security implication of having an unlocked boot loader?
- Why does unlocking the bootloader wipe phone?
You might also want to check other questions tagged bootloader-lock, there might be one specific to your device.
