You have successfully installed MODX Revolution 3.0.5-pl!
Now that MODX is installed you can login to the manager to create your templates, manage content and install third party extras to add functionality to your website.
Templates contain the outer markup of any page. Each resource can only be assigned to a single template at a time. By adding Template Variables to a template, you can add custom fields for any resource using that particular template.
With Chunks you can share parts of the markup, such as a header, across different templates. Snippets are pieces of PHP that return dynamic content, such as summaries of other resources or the current date. With snippets, you will often use Chunks to mark up the pieces of content it returns, instead of mixing the PHP and HTML.
Finally, Plugins enable more advanced features by hooking into the extensive events system provided by MODX.
To learn more about MODX, be sure to check out the Getting Started section in the official documentation.