Hello, and thank you for using *our* plugin.
You are right, messages are not very clear. Anyway here a short explanation.
Nginx keeps only one version of page in the cache. If your site has a "mobile version" that is served when a mobile device connect two things can happen:
1) The page is cached and the cache was generated by a request from a desktop browser. In this case the user who get the desktop version of the site instead of the mobile one
2) The page is not cached. In this case the user get the mobile version of the site and the mobile version also get stored in the cache thus when a desktop user connect, if the page is still cached, get the mobile version
If you flag the checkbox when the backend send the response for a mobile request set the HTTP header X-Accel-Expires to zero. In this way the response is not cached by NGINX
I hope these few lines help. Feel free to ask more question
 You can have multiple versions of the same page in the cache but you have to correctly set the the _cache_key_
 To determinate if a request come from a mobile device the plugin use the regexp you set in the next input field