For those unfamiliar with a bootloader, it is a program that controls which applications will run when you boot up your phone. To keep the process of powering on your device the same each time, and to keep out malicious software, the bootloader is usually locked by the manufacturer. But with the large number of Android users who want to add custom software on their phone, some manufacturers have decided to allow for an unlocked bootloader on newer models and are offering tools to unlock older handsets. Of course, we should let you know that if you turn your phone into a brick once the bootloader is unlocked, your warranty becomes useless.
If you have one of the HTC devices with a bootloader that can be unlocked, or you are unsure if your model is included, head over to htc.dev/bootloader. From that page, you will be able to view a list of all HTC models that can support the unlocking process and you can start the procedure. If your HTC phone is not on the list, you might want to try Xda's HTC Super Tool v3 which, as we recently reported, is theoretically able to unlock the bootloader on all HTC phones.
source: HTC via AndroidCentral