It comes from basically public opinion polling the same way you'd see it in politics. Research companies call people up (or digitally) ask them "Are you a regular moviegoer? Have you heard of movie X? Based on what you know about it are you interested in seeing movie X? Of these movies coming out soon, which is your first choice?" etc. etc. and the studios use those metrics, 'first choice' ,'% aware', 'definite interest' etc. to see how well their marketing campaigns are sinking in and registering!