音、くん - ON and kun

Many Japanese kanji have multiple pronunciations, and they each fall into one of two categories: the ON reading and the kun reading.

音

This symbol is ON. The ON readings originate from the original Chinese character that the Japanese character is based on. The ON readings are typically represented in English in all capital letters, or in Japanese by Katakana characters.

くん

This word is kun. The kun readings are native to Japanese, and are typically represented in English as lowercase letters, and in Japanese by Hiragana characters.