Recovery: Every Android device comes with a recovery through which the OS can install over-the-air updates. It i s also used by the system whenever you factory reset a device. If you want to flash a custom ROM, you will have to replace the stock recovery with a custom one.
Superuser/SuperSU: This application is automatically installed on the device during the process of rooting. It is used to grant or deny root access to other apps.
TWRP/CWM: Both are custom recoveries that allow users to install custom ROMs, format certain partitions or make nandroid backups.How to Flash the TWRP Recovery Environment to Your Android Phone (Team Win Recovery Project or TWRP)
Custom ROM: A customized and advanced version of Android containing many features and optimizations, usually based on the latest version of Android.
Bootloader:Is a the code that runs even before the operating system on your device boots. It helps in booting other operating systems by systematically loading their files in the memory. The bootloader of your Android device can either be locked, unlocked or unlockable.How to Unlock Your Android Phone’s Bootloader
Usually, you will need to unlock the bootloader of your device before rooting it.Rooting a device is a process of gaining full control of the device's OS and the software that runs it.While you can customize Android to suit your taste, device manufacturers add layers of software on top of the operating system hence limiting a user's options.
Rooting is a way to bypass this barrier and exploit your device's full potential. Rooting is the equivalent of jailbreaking an iPhone