Refinements are labels that you apply to web sites in your search engine index to help your users more easily find the information they are looking for. When a user clicks a refinement label, sites that you have annotated with the label are either searched exclusively or given priority in the results—you choose - developers.google.com 
# About Refinements
For example, say you've created a search engine about primates. Some of the sites you've added are specifically about chimps, and some are about gorillas. You can use labels to categorize the content on your site so that users can filter search results.
For example, say your Monkeys search engine includes content about gorillas and chimps. With labels, you can:
Example of adding refinements to Goolge Custom Site Search - developers.google.com
Annotate search results (for example, by labeling a result "Chimps" or "Gorillas"). Other useful labels might include "blog" and "official website". Enable users to prioritize or filter search results according to label.
# Creating Refinements
To enable refinements in your search engine, you'll first need to create a set of labels, and then apply those labels to sites you've included in your search engine.
You can create refinement labels and tag sites with them in the Refinements Tab of the control panel, but you can have greater control over the ranking if you are using the Custom Search XML Annotations file.