The browser will "install" Geo5, to be more precise: the used ressources: javascript files, pictures,...
What is changing for the user: If you start Geo5 the browser will load it from the local cache instead using the internet connection.
This is especially usefull for mobile devices with limited network speed.
Quote from wikipedia https://en.wikipedia.org/wiki/The_cache_manifest_in_HTML5
The cache manifest in HTML5 is a software storage feature which provides the ability to access a web application even without a network connection.
If you are technically interested look at http://www.html5rocks.com/en/tutorials/appcache/beginner/