Here is some more about what to expect from these Youtube interviews as provided by CNN White House producer Cassie Spodak:
Should last about 45 minutes
Interviews will take place consecutively, hosted by Google’s Steve Grove
President Obama has participated in post-SOTU Google/YouTube hosted interviewers before. The difference this year is that the interviewees are traveling to the White House (instead of interviewing him from home). In 2013 questions came from popular users of social media but 2014 and 2012 seemed to involve just your average American. This year, one of the users has more than 8M YouTube subscribers, the other two have around 3M and 2M.
2010 – YouTube interview, submitted interview questions
2011 People submitted question in text and video, and could vote online
2012 President Obama answers citizen questions in a Google+ hangout
2013 President Obama answers citizen questions in a Google+ hangout
2014 President Obama join a "virtual road trip" on Google+
In June of 2014 the President also participated in a live Tumblr Q&A not related to SOTU.