Assets loaded from a web server are usually cached. This can cause an issue when a file is updated, but the browser cache prevents the user from seeing the changes. The way to solve this but still enable caching, which greatly increases site performance, is to append a hash to the end of an asset's file name. So that app.css becomes app-jid09j1230123.css, and that hash updates every time the file does.

There are tools like gulp-rev that does this automatically when building the project and outputs a manifest file to track the changes. This plugin reads that manifest file and returns the correct file revision. So that assets can still be worked with easily:

{{ "assets/css/app.css"|revision }} ... returns "assets/css/app-jid09j1230123.css"

For now the name of the manifest file can't be configured so adjust your build system to save the manifest file as "rev-manifest.json" in your theme root.

If the filter doesn't find a corresponding entry in the manifest it simply returns what the standard "theme" filter would have returned.