I had to look it up. I think it’s this:
http://abcnotation.com/about
I don’t really understand the utility in today’s modern state of word processing. Feels easier to just include the sheet music, rather than forcing the learn a new way to read sheet music. Back in the mid-90s, I feel like it could have made sense.
ABC is a very useful "shorthand" and can also be used as a computer programming code!
Scenario : In a social environment and someone's playing a tune on an instrument you understand. They play a tune you rather like but have never heard. Grab a beermat, borrow a pen and scribble down the actual notes they're playing viz :
D A A A B/ c A G E D D
Doesn't look much, but it's the first few notes of "Suzy McGuire" (which happens to be the name of a girl I used to live with, but that's another story)
When you get home you can type these "letters" into your computer, along with a little other information, and produce a piece of sheet music to print out or read on screen.
If you then decide ... and this is the clever bit ... that you'd actually like the lowest note to be C rather than D, 'cos it suits your ukulele better or whatever, you simply ask the computer program to change the key and
*lo and behold* your sheet music is in a different key
Similarly, if you've got a sheet of music in an "awkward" key, it's the work of a few minutes to "copy-type" the music into ABC format, then get the computer to change key for you and produce a new sheet of music
You can also use ABC to generate MIDI tracks for use as accompaniment for practice
Once you've mastered the basics of reading music, ABC can be a really useful tool! What it isn't is "another way of writing music" that you're expected to sight-read ... though some people do!
Good luck :music: