Everyone has answered this really well, but I'd like to add some points that I wish someone had told me when I began learning.

The On-yomi is taken from the original Chinese reading of the kanji, and the Kun-yomi is the Japanese reading that has "adopted" the kanji, so to speak. As a general rule, if the kanji is by itself, (as in, it is not attached to other kanji, just hiragana) it generally takes on the Kun-yomi; if the kanji is part of a compound, it (and the other half of the compound) takes on its On-yomi.

Should I bother learning the non-preferred reading?

The answer is "yes". It's the difference between guessing the meaning of a word and being able to read it out loud. That may not sound like much, but it's absolutely essential.

For example: 食べる is pronounced with its Kun-yomi, たべる. 事 on its own is generally pronounced こと, which is the Kun-yomi. If if you combine them 食事, they're both read with their On-yomi: しょくじ

Again, that's very generalized and not always the case, but it's the rule rather than the exception.

Everyone has answered this really well, but I'd like to add some points that I wish someone had told me when I began learning.

The On-yomi is taken from the original Chinese reading of the kanji, and the Kun-yomi is the Japanese reading that has "adopted" the kanji, so to speak. As a general rule, if the kanji is by itself, (as in, it is not attached to other kanji, just hiragana) it generally takes on the Kun-yomi; if the kanji is part of a compound, it (and the other half of the compound) takes on its On-yomi.

Should I bother learning the non-preferred reading?

The answer is "yes". It's the difference between guessing the meaning of a word and being able to read it out loud. That may not sound like much, but it's absolutely essential.

For example: 食べる is pronounced with its Kun-yomi, たべる. 事 on its own is generally pronounced こと, which is the Kun-yomi. If you combine them 食事, they're both read with their On-yomi: しょくじ

Again, that's very generalized and not always the case, but it's the rule rather than the exception.

Everyone has answered this really well, but I'd like to add some points that I wish someone had told me when I began learning.

The On-yomi is taken from the original Chinese reading of the kanji, and the Kun-yomi is the Japanese reading that has "adopted" the kanji, so to speak. As a general rule, if the kanji is by itself, (as in, it is not attached to other kanji, just hiragana) it generally takes on the Kun-yomi; if the kanji is part of a compound, it (and the other half of the compound) takes on its On-yomi.

Should I bother learning the non-preferred reading?

The answer is "yes". It's the difference between guessing the meaning of a word and being able to read it out loud. That may not sound like much, but it's absolutely essential.

For example: 食べる is pronounced with it's Kun-yomi, たべる. 事 on it's own is generally pronounced こと, which is the Kun-yomi. If if you combine them 食事, they're both read with their On-yomi: しょくじ

Again, that's very generalized and not always the case, but it's the rule rather than the exception.

Everyone has answered this really well, but I'd like to add some points that I wish someone had told me when I began learning.

The On-yomi is taken from the original Chinese reading of the kanji, and the Kun-yomi is the Japanese reading that has "adopted" the kanji, so to speak. As a general rule, if the kanji is by itself, (as in, it is not attached to other kanji, just hiragana) it generally takes on the Kun-yomi; if the kanji is part of a compound, it (and the other half of the compound) takes on its On-yomi.

Should I bother learning the non-preferred reading?

The answer is "yes". It's the difference between guessing the meaning of a word and being able to read it out loud. That may not sound like much, but it's absolutely essential.

For example: 食べる is pronounced with its Kun-yomi, たべる. 事 on its own is generally pronounced こと, which is the Kun-yomi. If if you combine them 食事, they're both read with their On-yomi: しょくじ

Again, that's very generalized and not always the case, but it's the rule rather than the exception.