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. AppleSingle
is an alternative format to accomplish the same end, combining all the forks plus a metadata header in one file instead of keeping separate files like AppleDouble. While both AppleSingle and AppleDouble were introduced for use with early Unix-based Apple systems, AppleDouble survived as the main method of storing files with resource forks on OS X systems if a filesystem is used that doesn't directly support such forks.
The AppleDouble format keeps the data fork of the file in its original format and filename (this is the main file, as used by non-Mac operating systems, and for many file formats, the only one that matters), and creates a second file with the resource fork as well as Finder metadata. The second file has the filename of the main file with "._" (a dot and an underscore) preceding it. If it was encoded for transmission, Base64 was generally used.
Format detail links