Custom fields are a powerful feature for WP because they allow you to add arbitrary data to a post.
A decent place to start learning about custom fields is the Codex page on custom fields. Particularly the "Usage" section if you're just getting started.
Via the form described on that Codex page, you can enter custom fields for a post. A custom field consists of a "key" (some sort of identifier for the piece of information you are entering, to be used for later retrieval) and a "value" (the actual data you are trying to store). One such key value pair could be: "Price" and "34.00".
WP facilitates defining the custom field values, but only helps with basic code-based retrieval of those custom fields. The Get Custom Field Values plugin facilitates custom field retrieval and display via:
* Template tags (code-based functions; requires customizing theme template files to make use of the functions)
* Widgets (the most user-friendly way of retrieving custom fields)
* Shortcodes (to display the value of custom fields within the body of a post; comes with a shortcode builder so you don't have to remember any syntax)
I hope that helps you.