On this question, you can see this English explanation
Most proxies, most notably Squid up through version 3.0, do not cache resources with a "?" in their URL even if a Cache-control: public header is present in the response. To enable proxy caching for these resources, remove query strings from references to static resources, and instead encode the parameters into the file names themselves.
Generally, JS code or CSS code is called. The later version number can help cache refresh quickly. If your JS or CSS code no longer needs frequent changes, you can consider not increasing the version number.