Could you decipher the structure of the font file?

[Pugachov-Alex]

Hello.
Could you decipher the structure of the font file?
I can't understand what "Offset size" means, and there are two arrays below - "Width" and "Offsets".
I guess it's something for a proportional font, but I don't understand specifically ;(
(I want to make a font for CP1251).
Yours sincerely, Alexander
alex_pug@bk.ru