What is kerning?
Is a spacing between pair of letters, for example: if we have a word - "Kerning", the spacing between pairs of letters, like: "ke", "er", "rn", "in", "ng", it is kerning.
Good and bad
There is a bad kerning and a good one, simply, if a word looks out of balance, or some spacing between letters is even a slightly bit off, then our concious reacts to that, similarly like to a wrong color chosen or unbalanced, horribly aligned design.
Good kerning is when whole word or set of words looks even, consistent, balanced and overall has great kerning. It doesn't mean that a word has to be kerned mathematically, no, in most cases it is best kerned by designers intuition. Intuition is trained, like all the other stuff that designers see or know.
Examples given here are pretty rough, you will barely see such horrible kerning anywhere in typography, usually it's way harder to notice where good and bad kerning is, you must train your designer's eye for that.
You can learn it!
Kerning can and must be learned, in my opinion it is best learnt by training your designer's eye. When I was bad at kerning, I thought like: "What the heck, everything looks okay to me..." I just couldn't see a problem to solve, but now I do, of'course I still make mistakes from time to time, but I fix them as soon as I notice them.