Unlocking your Bootloader

What is "Bootloader Unlocking"?

Every Android phone has something called the "Bootloader"
This is what loads Android (or another OS), or your recovery just to name a few.

By default on most devices, the Bootloader is locked, which is done so that nobody can modify your device without permission.

How do I unlock my bootloader?

Disclaimer:

• Unlocking the bootloader WILL factory reset the device!
  make sure to backup any important data beforehand!
• Some OEMs implement techniques to detect if the device has been unlocked even after locking it again, for instance Sony with their DRM keys.
  I would look up the XDA Forums Guides section for your device to see if there is a way to bypass these, to retain more functionality after unlocking

This is different from device to device, though generally it goes something like this:

Prerequesites:

Make sure you've enabled "OEM Unlocking" in the Developer Options page.
Find out where to enable Developer Options

Unlocking Process

1. Visit your Manufacturer's bootloader unlock page
2. Enter any details it asks for, like an E-Mail or your device's IMEI number
3. submit your request and wait for a reply
4. You should now have an unlock code which can be used to unlock the bootloader
5. run fastboot oem unlock <your code here>
6. you should now see an unlock prompt on your device, asking if you would really like to unlock the bootloader
7. Hit "Yes" and wait for the device to reset

After that has been done, you can generally start modding the device!