To install custom ROMs, the simplest way is when you have an unlocked bootloader that doesn’t verify the software on your phone. Bootloader is like a start up manager, that checks different aspects of the OS to make sure it’s legit and controls the different partitions of device’s memory. An unlocked bootloader allows simpler installation of custom ROMs, that can change the whole experience of using a mobile device. The downside is that users who aren’t experts in such fields, can harm their device that way, so many manufacturers decide to lock their bootloader.
HMD is one of them, and after a number of questions asked by consumers, and a little internal discussion, HMD decided that they will unlock the bootloader on their device, one device at a time. HMD Global’s Chief Technology Officer Mikko Jaakkola confirmed that via Twitter: