I think it means the variant cover's printing ratio. Like, in this case a 1:100 cover means that for every 100 copies with regular covers that were made, only one with that variant cover was made. The comic book shop that initially got it would've had to buy 100 of the regular book in order to get 1 of the variant.
Sometimes books have multiple variant covers; there might be a 1:10 cover, a 1:25 cover, a 1:50 cover, and also a 1:100 or even 1:300 cover. Generally speaking the higher the number the rarer it is and the more valuable it is considered; at least, valuable to the sort of people who value that sort of thing.