Preferred Sites is an extension of Google SearchWiki, the feature for Google Search that lets you personalize the results by adding a list of sites, whose results you prefer to be a source of your search results seems to reflect their move to "Trust" ranking. The feature is an experimental feature that can be found in Google's preferences page if you are logged in and you were selected to test it.
"The preferred sites feature lets you set your Google Web Search preferences so that your search results match your unique tastes and needs. Fill in the sites you rely on the most, and results from your preferred sites will show up more often when they're relevant to your search query," Google explains.
Source:→ Google System