"Certified" means they have signed Google's Mobile Applications Distribution Agreement, a confidential contract where Google permits access to Google Apps in exchange for the manufacturer agreeing to load up all of Google's required bloatware, set Google Search as default, comply with all of their compatibility requirements, and agree to not release any devices running alternative versions of Android.

We don't have a current version of the MADA public, but you can see some old ones from 2011 here: http://www.benedelman.org/news-021314/