A cache buster is a string of code inserted in a page or a tag to prevent browsers and proxies to fetch a file (most often an ad) from the cache (see more details at cache busting).
All ad tags must have a cache buster.
Two forms of cache buster code are mainly used, random numbers and time stamps. Due to the cache buster, for each new request of the same page, the browser considers the ad tag as a new one.
An example of cache buster using time stamp: