The Zimmerman acquittal is about preserving a pernicious, but deeply embedded, cultural image in America that young black men are, almost by nature, criminals. It isn't true, of course; high crime in inner city neighborhoods where many black people have no choice but to live is due to poverty and other factors, not race. Yet this stereotype exists and must be knocked down.
Trayvon Martin, an innocent minor, was killed because some paranoid, armed, wanna-be cop saw him and immediately, unthinkingly thought he looked like a suspicious trouble-maker — even though all Trayvon had done was go to the 7-11 for Skittles. Every white person out there knows what I'm talking about, if they are honest and candid about it: if they see a black man in a white suburb, they immediately feel uneasy and suspicious and assume something is wrong. It’s a form of automatic, emotive racial profiling. But you know what? Most actual gang members know enough not to go to nice neighborhoods, because the policing is too good. So it's really unlikely that your nice subdivision with the green lawns and picture book houses is about to be overrun by crime just because a young black man shows up.
The stereotype of the black male as "gangsta" is believed by many non-black people on an almost subconscious level. It isn't racism like Nazi racism, where there is a doctrine that explicitly dehumanizes a group. It is instead a cultural racism, an idea permeating the culture based on a prejudicial stereotype against an entire group. It is a key part of the worldview of (mostly white) conservatives, and indeed helps define their identity and gives them a (false) sense of superiority. The Trayvon Martin case has been such a major media event because it is in part about protecting the ability to continue believing these stereotypes, and to keep up the profiling of people based on race.
This image of black criminality ubiquitous in the sensationalist media, and is promoted by conservative politicians and demagogues because it appeals to the fears of their voting base: fears which caused "white flight" to the suburbs that maintained de facto segregation; fears which keep little old ladies locked behind closed doors and security systems despite living in safe neighborhoods; fears which feed the false sense of victimization of privileged whites; fears which fuel the American gun culture; fears which have led to these ridiculously preemptive "Stand Your Ground" laws; fears which keep working people from unifying, regardless of skin color, to stop the rich from exploiting the poor and squeezing the middle class.
If you're not black, and when you see a black person in your neighborhood you immediately become fearful and suspicious, you are helping to make a world in which the mothers and fathers of kids like Trayvon Martin have to suffer the grief of needlessly losing a child. You might not know you are, but you are. So if you're a white person who lives in a nice subdivision, the next time you get uncomfortable when you see a young black man or any other person of color there, pause to think about it for a minute: really, ask yourself, is that a criminal, or is it just a person going about their legitimate business? I bet if you talked him or her, you'd find out that they're really just another human being, not a monster.
Pausing to think like this is the first step to overcoming prejudices – because stereotypes flourish where thinking stops. It would at least be a start.