This might be the silly question but am bit new to Android world so I wanna know what does porting of rom means?wanna know in detail
Porting a ROM is basically having the features of one devices to another devices. By using a Custom ROM you can add software capabilities your device doesn't have provided your phone's hardware supports it.
For example, if your phone manufacturer ships your phone with KitKat and doesn't offer upgrade, you can use Custom ROM and upgrade your phone to Lollipop, MarshMallow etc. However, if your phone doesn't have an OTG hardware, you can have ROMS that support OTG but since your hardware doesn't support it you cannot have it enabled.
Making the ROM from scratch is difficult but Porting isn't too difficult.
Rare. That's what it is
ROM porting is the process of adapting one ROM to use on another device. For example, porting a Samsung galaxy S4 ROM to the S5. however, it is a long tedious process that takes wayyyy too long.