1.
on'yomi and kun'yomi; Chinese and Japanese kanji readings
The two types of readings for kanji characters: the on'yomi (Sino-Japanese reading derived from Chinese pronunciation) and the kun'yomi (native Japanese reading).
音訓 is a compound of 音 (sound; on'yomi) + 訓 (reading; kun'yomi). It is an essential term for anyone studying Japanese writing.
音読み (on'yomi) is the reading derived from the original Chinese pronunciation, typically used in compound words (熟語). 訓読み (kun'yomi) is the native Japanese reading, often used when the kanji stands alone or with okurigana.
Example
The kanji 山 has the on'yomi サン (as in 山脈, mountain range) and the kun'yomi やま (as in 山, mountain).