The reading (読み) of a kanji character or a word written in kanji characters refers to how it is “read” in terms of kana characters. For example, kanji character 日 has two common readings にち and ひ.
There are two kinds of readings of a kanji character: on readings (音読み) and kun readings (訓読み). On readings are derived from Chinese pronunciations at some point in history, and kun readings are native Japanese words corresponding to the meaning of the kanji characters. In the above example, にち is an on reading and ひ is a kun reading of kanji character 日. Most kanji characters have at least one on reading and at least one kun reading.