AppleSingle is one of the systems used to store the Resource Fork of Macintosh files on filesystems not natively supporting it, something which became necessary when Apple moved to Unix-based operating systems instead of "classic" MacOS. AppleDouble is an alternative format to accomplish the same end, using two files where AppleSingle uses one.
AppleSingle combines the data and resource forks with Finder meta-information into a single file, similar in concept to MacBinary.
AppleSingle files begin with signature bytes (hex)
00 05 16 00.
Refer to AppleDouble#Specifications.