Naka vs. Chyu (Japanese); clear error in text display.
There is a repetitive error in the Japanese course wherein the hiragana for chyu is inappropriately substituted for the hiragana for naka. This appears to be in every single lesson. The audio and context give naka, but invariably chyu appears. It is absolutely maddening and clearly a programming error and not an intentional choice by the course designers. Somebody should tweak that out.
Here's an explanation for the difference between on'yomi and kun'yomi. In short, kanjis derive from the chinese characters hanzi (most of them have the same meaning), so the japanese, instead of just keeping their way of saying things also added the chinese one into teir langage and then it became super complicated. In short :
"On'yomi 音読み: Readings derived from the Chinese pronunciations. Kun'yomi 訓読み: The original, indigenous Japanese readings." (extract from the article)
Example : 山 can be read the japanese way (やま), or the chinese way （in chinese shān i believe, translated さん in japanese).
There is kind of a general rule (althought not 100% accurate) to when to use which pronunciation, but check the article for it (and the blog, it is really awesome, it's the same crew than WaniKani and there is a lot of tips regarding learning japanese or japanese culture).