Manual talk:$wgResourceLoaderMaxage/Flow

What does versioned mean here?

It refers to whether the load.php request url contains a version query parameter. All requests generated by mw.loader in JavaScript are versioned. The only unversioned requests are: 1) the startup module, 2) the stylesheet. Both are referenced from cacheable page HTML. Versioned responses never change and may be cached for a long time. (Whenever their modules change, the version changes too and therefore the url won't match). The unversioned response from the startup module is relatively small and should be cached for a short period of time to ensure deployments and upgrades apply quickly. This then bootstraps the front-end environment and all other modules loaded after will use the latest versions and are cacheable for much longer, given they are loaded from versioned urls.

See also ResourceLoader/Features.

Thanks for the detailed reply. This made our site load much faster :)