what is a bootloader unlocked phone? What is a bootloader unlocked phone? 
How we can make use of it while creating or flashing ROMs?
What are advantages of unlocked bootloaders over locked bootloaders?
Why does not any OEM generally make bootloader unlocked phones?

I am pretty naive in this topic. Thanks in advance.
Every Android phone has a bootloader that instructs the operating system kernel to boot normally. But you need to understand one thing here that as Android OS is an open source OS and is available on a variety of different hardware, every manufacturer has their own version of bootloader specific for the hardware present in its environment. (...) A bootloader is usually locked on an Android device because although it’s an open source OS, still the manufacturers want you to stick to their Android OS version specifically designed for the device. In order to apply this concept, manufacturers lock the bootloader.


(Source)

Essentially, the advantages of having an unlocked bootloader for your device is that the manufacturer of that device has granted you the right to modify the operating system (Android) present on the device.

It's interesting to note that the term "unlocked" bootloader would seem absurd if we were talking about a PC; the freedom to boot any OS you want has always been taken for granted on that platform, but because locked bootloaders are so common in the mobile world for many different reasons, it has become something odd for the bootloader to be "unlocked".

The reason that bootloaders are locked on some devices are fairly obvious: the manufacturer of the device wants to be in control of what software is running on it. Since devices are often locked and tied to a contract, having control of what code runs on the device is essential. It's also a question of support; if you were the manufacturer, you wouldn't want to handle support calls from people all over the world who had accidentally bricked their phones by flashing corrupt software onto their devices. In a sense, locked bootloaders protect (some) users from themselves. Depending on your point of view, this can be good or bad. For a corporate customer buying devices for employees, access to the bootloader probably would not be high on the wish list.

Please let me know if there's anything in this answer that is unclear and I will try to elaborate.