kanji dictionary; Chinese character dictionary
A dictionary focused on Chinese characters (kanji), explaining their readings, meanings, and often stroke order. Distinct from a general word dictionary.
A broader sense, used synonymously with 辞書 (じしょ). In modern Japanese, 辞書 is far more common for a general dictionary; 字書 can sound slightly old-fashioned or formal.
See also: 辞書
This kanji dictionary also explains the origins of the characters in detail.
If there's a word you don't understand, please look it up in a dictionary.