People sometimes find the concept of on and kun readings of kanji difficult to grasp, and consider them terribly complicated and foreign. However, they have a rather exact parallel in English.
What I am about to explain does not directly assist us in learning Japanese, but some people have told me that it makes the whole concept of on and kun readings much more friendly and graspable, so I am presenting it to you in case you too find it helpful.
The truth is that differing readings are not unique to Japanese. Something very similar can be found in English, and it came about for very similar reasons.
Kun readings represent the original Japanese pronunciation of a word while on readings represent the more “learned” Chinese word that came into Japanese so long ago that it is now a completely naturalized Japanese word. On readings are more usually found in compounds and more “learned” or abstract words.
Exactly the same is true of English. English “on readings” are not Chinese but Latin (sometimes Greek), and because English doesn’t use kanji it is less immediately obvious, but when we take a few examples it becomes very clear:
Japanese: Kun reading mi(ru). On reading ken/kan (ex: 見物 kenbutsu = watching, sightseeing
English: Kun reading see. On reading vis (ex: vision, visible)
Japanese: Kun reading te. On reading shu (ex: 手動 shudou = manual, by hand)
English: Kun reading hand. On reading man (ex: manual)
Japanese: Kun reading hoshi. On reading sei (ex: 火星 kasei = fire-star = Mars)
English: Kun reading star. On reading stell (ex: stellar, constellation)
Japanese: Kun reading inu. On reading ken (ex:犬舎 kensha = kennel, doghouse)
English: Kun reading dog. On reading ken/kan (ex: canine, kennel)
Sometimes, of course, there are several readings in both Japanese and English. English words sometimes have on readings from both Latin and Greek. For example book has the on readings bibl (as in bibliography, Bible) and libr (as in library).
Of course this is somewhat fanciful, since English does not have kanji and therefore we do not really have the concept of “readings” in English. However, if Greek/Latin had had “kanji” (ideograms) and English had adopted them (as Japan did Chinese characters) we would probably have had an exactly parallel situation.
In many ways kanji make things easier. It is clear to see how the concepts expressed in a kanji are in fact the closely related despite pronunciation differences. The foreign learner of English is forced to learn book, library, bibliophile etc by “brute force” as unrelated words. Kanji make such relationships clearer.
But, as you see, on and kun readings are not actually something that is very foreign to English even though they work somewhat differently in the two languages.
I hope this makes them feel a little more approachable.