This file is a reminder of what each colour in the image's palette corresponds to.
It's not used by the program, but without it, I'd forget which is which.
Note that in the actual PNG file, the colour 0 actually codes the transparency.
GIMP ignores it and starts the palette indexing on the next colour, though, at 0.

0 : skin
1 : skin outline
2 : mane
3 : mane (darker)
4 : mane outline
5 : eyes
6 : eyes smaller
7 : eyeliner
8 : cutie mark
9 : cutie mark (darker)
10 : black
11 : white
12 - 13 - 14 : supplementary colours
15 : pupil (usually black)