[Note4] What is kernel? What is custom kernel? Why do I need it? (noob questions)

Hey guys, ive got some questions about custom kernels.

I've learnt something about custom roms and flashing them from Galaxy S2, Galaxy Note 2 and then Galaxy Note 4. Im not a programmer, im not an IT guy. I've debloated some roms by myself using Root and an app from Gapps to delete system files. And here my knowledge ends. Sometimes I've bricked my phone and I dont know why :).

Now I need to ask you few more questions about Note 4 kernels.

So :
1) What exactly is kernel?
2) What is the difference between custom kernel and stock kernel?
3) What are good things about flashing a custom kernel?
4) What I can change after flashing? I have also some profiles from some other threads on xda to use in kernel auditor but tbh i dont know what im exactly doing.
5) What kernel should I use (for 910F)? I really enjoy a long battery life (dont need a gaming performance, i use my phone to work and be online)
Please anwser me in a simple words without mumbo jumbo that I cant understand. Im sorry for my lack of knowledge :)

Thanks guys.

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