In most cases, kanji reading asks for the on’yomi and vocab reading for a single kanji asks for the kun’yomi. The on’yomi is the Chinese reading, and the reading you’ll use generally when you find several kanji together, while the kun’yomi is the Japanese reading, and one you’ll use more often when a kanji is alone.
That’s why, for example, 山 is read サン on the kanji reading (on’yomi), but やま on the vocabulary reading (kun’yomi). Yet if you find 火山 you read it かざん, using the on’yomi. In this case, サン would be mountain as a part of a word, while やま means mountain as a word. If you were to say “a mountain”, you’d say やま, but to say “mount fuji” you’d say ふじさん.
It’s a bit difficult to grasp in the beginning, but you’ll get a good grasp on it as you advance 