Films used to be separated into film reels, 1-x, could be 1-7 or 1-15 or however long it needed to be. Nowadays since film is not analog anymore, it's mostly kept up just arbitrarily for production.
1m1 means reel 1, music 1 - the first piece of music heard on the first reel. 4m3 means it's the third piece of music heard on the fourth reel. But this was not always consistently applied, sometimes it goes reel number+number of music cue in the entire film, so 2m32 would mean we're in reel 2 and this is the 32nd piece of music heard in the entire film. This latter is more problematic when the cut changes a lot - you cut out a scene and suddenly every piece of music after it would have to go down a number. In the first system I explained, if you remove the scene containing 3m3, 3m4 and 3m5 would have to go down, but 4m1, 4m2, ..., 5m1, 5m2, ... 6m1, 6m2... would still remain the same.
