It's neither confidential, nor is it an ID, nor does it identify you.
That number comes from jQuery, it is appended to the URL here and created here, and it is nothing more than the number of milliseconds that have passed since January 1st 1970, 0:00 UTC, according to your computer's clock.
This parameter is there because that AJAX request is made with jQuery's AJAX option
cache set to
false, to prevent your browser from showing you outdated cached data and instead force it to pull fresh content from the server. This goal is achieved by appending this meaningless (but unique) parameter to the URL, so to your browser it looks like it's a totally new URL that it has never seen before.
The server doesn't care about this number; it completely ignores it.