It's a way to identify distinct pitches that are usually treated as the same in equal temperament.
For instance, in just intonation 2 (the major second of the scale) has a frequency that makes a ratio of 9/8 relative to the tonic, but sometimes you might want a slightly flatter major second with a ratio of 10/9. So, that note is label 2- to distinguish it from the regular major second.
For instance, in just intonation 2 (the major second of the scale) has a frequency that makes a ratio of 9/8 relative to the tonic, but sometimes you might want a slightly flatter major second with a ratio of 10/9. So, that note is label 2- to distinguish it from the regular major second.