Ahh, since you have no knowledge regarding rooting and all I'll just explain in the simplest form. Imagine that you have bought a laptop from HP which Congress preloaded with a windows installation and some pre-stuff, I assume that a normal person would't be bothered by it and the extra useless stuff they install on your laptop. They prevent you from uninstalling them, blocks access to C directory and pretty much other things that some medium user would need. So what he does is install a new os, any os I mean windows 10 or 8 or linux. And so he removes all the bloats and gets acces to many things.
Now getting to the point, that to install a new os, or rom, in android's case, you first need to unlock it's bootloader. Now the bootloader does what it says and manages your boot preferences, just like the bootloader and bootmanager in laptops. As it doesn't need to be done in a computer or laptop, it is neccesary to unlock it to install a custom recovery for your phone.
Twrp is a custom recovery like cwm, and benefits of having a custom recovery are many, just like your windows recovery menu which let you reset your laptop or factory default it which just refreshes the window as it was just installed. So a recovery in a phone is just like the PC one, but having a custom recovery is necessary, as it provides a lot more features.
Now as got my meaning of bootloader and recovery, and if you still don't get something, you can just ask.
And now comes rooting as unlocking bootloader and installing a custom recovery doesn't need rooting your phone. Rooting your phone allows you to get admin privileges, just like in windows, and now you can even tamper with the internal files. You can root your phone with the android you got installed with your phone and don't need to install a custom recovery and all.
As I previously said that these things needn't be done for a laptop, but a phone needs it to be done in order to install a custom rom, or rather saying an unofficial update, or an official update which isn't available by some reasons. So that's why you need to do these things.
And by the way, could you tell us which phone are you trying to update, as then we can check and tell you if you can update without going through all this hassle.