Xiao’erjin (alternatively xiao’erjing¹ 小儿经) is the name of a form of transcription for Mandarin and related languages. Rather than using Cyrillic or Roman letters, the Arabic script is used. China has had a large Muslim population for about as long as there have been Muslims, and it was among those of them who were less likely to have a traditional classical education that the system was used.
The structure is fairly simple. Syllable initial consonants are written with a single Arabic letter. The final then was primarily done with harakat or vowel diacritics. Before Annals of Wu, was blogging on xiao’erjin and Chinese Islam in general on another site, appropriately enough called xiao er jing.