Google will look at who you link to, then use that information to add content that your custom search engine searches against.
Imagine, if you have a resource page of mobile products and you constantly update that page with more resources. In the past, if you had a Google CSE, you would have to manually add the new resource to your Google CSE sites list. Now, Google will automatically pick up that you linked to a new resource, and it will add it to your Google CSE automatically.
How do you do this?
Go to Google CSE, then click on your “control panel”, then click on “sites” and then click the “add sites” button. That will pop open this box:
Select the “dynamically extract links from this page and add them to my search engine” option.