It's just a "weight" for the random roll.
You add frequencies for all skins correct for desired time period, then make a roll out of the total and the chances for each of them are proportional with their correspondent frequency.
For example a skin with frequency 3 will have 3 times more chance to come up compared to one with frequency 1.
If we have
Skin A freq: 2
Skin B freq: 3
Skin C freq: 1
Total = 2 + 3 + 1 = 6 -> results in:
Skin A chance out of 100: (2 / 6) * 100
Skin B chance out of 100: (3 / 6) * 100
Skin C chance out of 100: (1 / 6) * 100
Sorry it's kind of hard for me to explain in english.
But I don't think it works if thats what you're asking
Can't remember might have been just wishfull thinking on my part when I wrote that. I like to write stuff for the future and my files are always full of this kind of stuff.
Dan