A cache-buster makes the URL unique each time the content is new and the content will therefore not be in the cache.
Because the cache key of the resources contains the URI, the HTTP cache mechanism (of the browser, proxy,…) will have then a cache miss and will be then forced to fetch the resources again.
The cache-buster can take the form of
my.js?ver=3.0.1
<!-- or -->
my-3.0.1.js <!-- with http rewrite rule to redirect to the file my.js -->