Search Engine Friendly URLs are paths to your page that make sense to the average human. If you don't have this option enabled in the Global Configuration for your site, you will have PHP generated URLs that won't be easy to read, easy to replicate and bad for SEO on your site.

To turn on this option go to:

System -> Global Configuration -> Site tab

sefurlSet this to Yes.

Troubleshooting

After enabling this option, check out your site. Navigate around to a number of different pages. You should see that the URLs for your pages (up in the address bar of your browser) show a logical path for the different pages. 

If you get an error page, it could be a number of issues.

1. Be sure that your htaccess.txt has been renamed to .htaccess in the root directory of your site.

2. In your configuration.php, be sure that the $live-site variable contains the correct path to your site.

public $live_site = 'http://joom3.zemplate.com';

3. Check out these two articles from docs.joomla.org that may help you find why your site won't work with the SEF URL option set to yes.

Enabling SEF URLs

Common problems when enabling Search Engine Friendly (SEF) URLs