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
当て字を使う
to use phonetic kanji
当て字をえらぶ
to choose phonetic kanji
「マジ」というスラングによく使われる当て字は「本気」です。
Phonetic kanji often used for the slang "maji" is the letter for "serious."
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.
日本語の当て字は、外国人にとって分かりにくいです。
Japanese ateji are difficult for foreigners to understand.
亜米利加 (アメリカ)のように、発音が同じ漢字を使う当て字の方法もありますよ。
Just like 亜米利加 (amerika), there's also a way of ateji where kanji are selected solely for their sounds and the meaning is ignored.