Phonetic Kanji
Ateji, Substitute Character
noun
当てる means "to guess" or "to hit", and 字 means letter. So these are guessed letters? Kind of, they're so-called phonetic kanji, or ateji.
This refers to cases where kanji are used solely for their sound, but not their meaning. There are also some cases of ateji where kanji are used only for their meaning and not their sound, but this is less common.
The readings are the kun'yomi reading for 当て and the on'yomi reading for 字. You've seen 当てる in the past, so you should be able to read this portion as well. Combine the two together and you have yourself a nice reading.
当て字をする
to assign phonetic kanji
当て字をつかう
to use phonetic kanji
当て字をえらぶ
to choose phonetic kanji
「珈琲」は「コーヒー」の有名な当て字です。
珈琲 is the well-known phonetic kanji for the word コーヒー (coffee).
この本は、当て字を多く使用しているため、少し分かりにくいです。
This book is a bit confusing because it uses a lot of ateji.
「先生、この当て字はどう読むんですか？」と聞くと、先生は少し青い顔になってこう言った。「その読み方は…知らないほうがいいです。今はまだ…ね。」
"Professor, how do you read this ateji?" I asked. She went slightly pale and replied: "You're… better off not knowing that pronunciation. Not yet."