To be honest, it would go way too far, to answer it in detail. There're a lot of ressource about it on the web. Just one thing - It has nothing to do with oxwall itself, but with the way your browser treats ressource like images. How long should it store something, until it refreshes it?
It is something you have to change within your htaccess file. That being said - If you never worked with those things, you might want to read a few articles and guides. Changing the htaccess not only can stop your site from working, but also can be a security issue, if someone is able to access your server files after you changed something wrong. So that someone can read your config.php with your database login details for example. Might not happen, but it can!
Google loves that browser caching, as it means, that a site gets loaded faster, if the ressources are already there and not have to be loaded. That also helps the server, as it lowers the requests, which again might help every visitor.