Whats this kernel stuff?

I've been interested in root and change ROM (if I can find one thats better in the battery department than stock).
So carefully following the plethoria of info, eg Darklords Note 5 ROM installation I thought I had it pretty well sussed.
I use Linux, so the preferred method to install stage 1/TWRP was by using Heimdall, which I found to be useless until I discovered that it needed to use a USB hub instead of the hubs direct out of PC. (There has been exact same errors reported in the forums about Heimdall which I seem to have solved by use of a USB hub.)
A double-check on install of Darklord ROM on youtube the user said very quickly and without detail to install the kernel. Well thats not mentioned in DarkLords installations info:
Installation Steps
- Full Wipe (System, Data, Davik Cache, Cache)
- Flash Rom
- Reboot and enjoy
What am I missing? If I just install the rom without the kernel is the stock kernel left behind and used or what?
Also a tad scarey I notice that if you choose the wrong kernal, you can end up with system corruption/boot error. But when the writer of a ROM does not make a recommendation how do we know whats going to suit?
Thanks vm.
Alistair.