A trim tab is a small device on the control surfaces of aerodynamic or hydrodynamic vehicles, such as ailerons or rudders. It is a second control surface attached to the first which serves to alter the hydro- or aerodynamic forces which would act on the control surface if the surface were completely straight with no tab. By manipulating the trim tab, the operator of the vehicle can effectively change the rest position of the rudder - the position the rudder would take where hydro- or aerodynamic forces would not force it to another position. The effect of a trim tab is that a small change in a small, seemingly insignificant section of the vehicle can have an extremely significant effect on the movement of the vehicle.
This makes for a very effective metaphor - the architect, inventor, and futurist Buckminster Fuller famously used it as a metaphor for how a single individual refusing to accept the status quo - the equilibrium point - can fundamentally alter the course of the metaphorical vehicle of humanity. This ties in well with his other famous quote - "I seem to be a verb," in which he describes himself as an "evolutionary process - an integral function of the universe."
Unfortunately, Buckminster Fuller did not live to see the day when biodomes were built using his model of the Geodesic Dome, or when sixty atoms of carbon were arranged into the same geodesic dome (referred to as Fullerene or "Bucky Balls"), intended to be used as a vehicle for transporting medication to specific locations in the body. We however, are fortunate enough to see that he was right in choosing his epitaph, "Call me Trimtab."
To return to another of Fuller's metaphors, that of Spaceship Earth - each individual on this planet is a trim tab. We appear insignificant but each one of us can have a significant systemic effect on the course of this Spaceship Earth if we are willing to stand and be counted. Mohamed Bouazizi, the street vendor who immolated himself and catalyzed the Arab Spring. Edward Snowden, who informed the international community that their basic human rights to privacy and security in their persons and communications were being systematically violated. Scientists, political dissidents, entrepreneurs, journalists (at least the real ones, anyway), anyone willing to take a position, to try to communicate something, to push ahead our understanding of the natural world or the human one, are all capable of having fundamental effects on the course of our lives as a totality.
As Friedrich Nietzsche said, "What is great in man is that he is a bridge and not an end." We are all trim tabs on Spaceship Earth. When there is a place we ought to go, or something that must be done, stand and be counted.