On’yomi (おん readings) are the original Chinese readings.
おんよみ is the original Chinese pronunciation of a kanji as it was imported to Japan.
Since kanji came from China, a lot of the pronunciations came with it. The おん reading is generally a close approximation of the original Chinese reading, similar to how katakana in japanese is a close approximation of foreign non-Japanese languages.
Kanji was introduced to Japan from China several times over a few hundred years. This happened because different dynasties in China would take over, and each dynasty had their own pronunciation of various kanji and would make it “official” when they took power.
When the Chinese would bring over the “new” pronunciation for kanji everyone already had, Japan would add another pronunciation to a bunch of old kanji. Because of this, kanji have multiple おん readings, and many of them are used. Luckily, for each kanji, most of them use a single on'yomi 80-90% of the time.
On’yomi works like this:
You have a single kanji, and it has a pronunciation.
From here, you take these kanji and combine them with other kanji, forming jukugo.
Jukugo are multi-kanji compound words.
Since on’yomi readings of kanji are essentially imitations of the Chinese readings of kanji, when you see jukugo, there is a good chance that this is a Chinese word, imported to Japan.
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