ABC is a tune notation format. It's mostly intended for modern, Western music, i.e. 12 semitones/octave, bars, time-signatures, etc. It's used quite widely in the folk music world. It uses source files that are human-readable and platform-independent, so they can be edited with any text editor and exchanged via email. There are lots of systems available to typeset tunes, play tunes through speakers or to an audio file, manage tune collections, etc. Chris Walshaw's abc page is a good place to start, with an explanation of the format and links to software and tune collections.
Many of the Monday Musicians sheets were typeset using ABC, so where possible we've provided the ABC source in the archive in case it's useful. However, please note that:
ABC files are available for 1252 = 73% of the 1725 tunes (although 79 = 5% are hidden). The visible ABC files are available as a single ZIP file (726.7 kB).
Go to the tunes listed by title or by sheet.