/(.)
: is the group of variables for which one Rewrite is applied. With this (.) group Rewrite; will be applied for all of the possible variants which can appear after / in example.com/;/media/
: is the directory, where CDN servers will go for the content;$1
: is the number of the group of variables. If you have more than one group, the next numbers will be 3 and so on.