You need a notification service, and google has something like that for us…

how does this works??

Take a look at the image below,

you need to register your android app in the google service, and your web interface will need an id, so everytime you want to push something to the android, your web interface instead will push it to the google server with the Id of the app, then google (no matter how) will localize your app, and even if its not running, they will get the notification,

behind the scenes there is a couple of thing that you must do, bu nothing like launching rockets from the NASA.

I will suggest to take a look to some tutorials
in order to start with the registration of your app, get the api key etc etc..