The new area code was approved by the California Public Utilities Commission back in December 2013 as the relief method for the existing NPA 415. Area code 415 covers the County of San Francisco and parts of San Mateo and Marin counties, with cities such as Inverness, Mill Valley, San Francisco or San Rafael. The new area code will serve the same communities.
A permissive 1+10 digit dialing period, during which calls in the area may be dialed using both 7 and 1+10 digits, began on August 16, 2014 and will end on February 21, 2015 when 1+10 digit dialing becomes mandatory. The new area code 628 will be active on March 21, 2015.
See official NANPA announcement and map of the new overlay complex (PDF document)