The Silent Majority are the people who are in a Fandom but don't feel the need to speak about how they like the work they're a fan of to the internet, or even in real life. Both senses of "how" - they don't talk about their fandom, and they don't say exactly what it is that makes them a fan. Their silence keeps them out of the Unpleasable Fanbase; their attitudes can be learned only by implication, by measuring the difference between open comment and Ratings and making educated guesses about what's in the gap.
These people are important. They still buy books, music, video games, and movie tickets. They still get and fill out Nielsen diaries. They are the ones who keep shows alive years after most on the Internet are thinking "Hey, is that still on?" They are the ones who support works that are popular despite open internet-fandom contempt. And if creators who are Pandering to the Base have no idea that they are part of the base and no idea what their probable attitudes are, the pandering may backfire—and none who actually speak of the work will ever be sure why...
Though then again, this all assumes that the content has a silent majority that silently enjoys it in the first place. Many a Small Name, Big Ego or unrestrained creator has operated on the idea that everyone at odds with their content is part of a Vocal Minority or another, and that the masses would enjoy it regardless of the issues brought up by said minority... only to be unpleasantly surprised when they find out the silent majority started leaving in droves or never cared to begin with, often due to aforementioned issues.
As a broader political term, the Silent Majority is an unspecified large majority of people in a country or group who do not express their opinions publicly. The term was popularized by the U.S. President Richard Nixon in a 1969 speech ("And so tonight to you, the great silent majority of my fellow Americans, I ask for your support"), where it referred to those Americans who did not join in the large demonstrations against the The Vietnam War at the time, who did not join in the counterculture, who did not enthusiastically participate in public discourse or the media and, most importantly, still supported him. Nixon saw this group as being overshadowed by the more vocal minority. Turned out he was wrong — at least with respect towards majority public opinion towards Vietnam, which was growing increasingly hostile and would continue to do so throughout his presidency (members of the counterculture, of course, by definition tend to be somewhat in the minority, otherwise it would cease to be the counterculture and simply be the 'culture').
This majority referred mainly to the older generation (those World War II veterans in all parts of the United States) but it also described many younger people in the Midwest, West and South, many of whom did eventually serve in Vietnam.note The Silent Majority was mostly populated with the blue-collar people who allegedly didn't have the ability or the time to take an active part in politics other than to vote. They did, in some cases, support the conservative policies of many politicians. Others were not particularly conservative politically, but resented what they saw as disrespect for American institutions. The term "Silent Generation" was used to describe the generation between the WWII one and the Baby Boomers (thus, mid-1920s to 1944 births, basically anyone old enough to have been alive during the war but too young to fight in it) - this generation now forms the bulk of the Tea Party and is the only living American generation to average out as more conservative than the one before.
The silent majority theme has been a contentious issue amongst journalists since Nixon used the phrase. Some thought Nixon used it as part of the Southern Strategynote ; others claim it was Nixon's way of dismissing the obvious protests going on around the country, and Nixon's attempt to get other Americans not to listen to the protests. Whatever the rationale, Nixon won a landslide victory in 1972, taking 49 of 50 states, in many ways vindicating his "silent majority."note The term nowadays in modern political discourse refers to supporters of Donald Trump that fuelled his rise to power in the 2016 election.
Contrast with Vocal Minority.