The word “solidarity” — basically, agreement between and support for members of a group — is not hard to define. But it can be hard to wrap your mind around in a world more oriented toward personal development and individual success than the common good. People who are willing to sacrifice their own freedoms or bodily security for someone else are celebrated in our culture, but also viewed with a bit of suspicion. What game are you really playing? What do you actually stand to gain?

“The Strike” (on the PBS app and PBS YouTube channel), directed by JoeBill Muñoz and Lucas Guilkey, is on its surface a documentary about the practice of solitary confinement in America. It centers on a series of hunger strikes organized by incarcerated men at California’s Pelican Bay State Prison, beginning in 2011, in protest of conditions in the highest-security prisons. This included protracted periods of isolation for individuals suspected of being in gangs, during which, inmates said, they were given inadequate food, denied meaningful contact with the outside world and held for periods that could last decades. (Under the “Mandela Rules,” the U.N.’s standard for solitary confinement is 15 days; more time is considered torture.)

The inmates also objected to a policy requiring them to “debrief” — that is, to provide information about gangs to authorities — in order to be released from solitary. Some of the formerly incarcerated in the film say they were identified as gang members simply because of the materials they read, or because of their race, without proof. And once you were in solitary, it was almost impossible to get out.

“The Strike” focuses on a number of former inmates who spent prolonged periods in solitary and participated in the 2011 hunger strikes. Two years later, with little to no change occurring, inmates called for another strike — and at the start, nearly 29,000 inmates refused food, across two-thirds of the 33 California prisons and four private out-of-state prisons holding California inmates. The 2013 strike lasted for two months, and by the end 100 prisoners were still refusing food.

Among the remarkable stories told in “The Strike” is how incarcerated people in isolation could organize a strike in the first place, as well as the men’s stories of life inside, and later outside, the walls of Pelican Bay. One technique involved emptying the water from the toilets in their cells, then shouting through the commode, where they could be heard by other inmates.

But it’s hard to ignore the other story here, one that illustrates both the meaning and power of solidarity. For the strike organizers, this was an obvious necessity almost from the start, in 2011. They were men, the documentary participants explain, who had been taught to hate one another all their lives — rivals from different neighborhoods, different ethnic groups, people with warring loyalties.

Collective action can only work when the actors are united — a big reason authorities attempted to divide the prisoners, promising unlimited food to whoever would break the strike. So, the leaders decided to sign a nonhostility pact — a “show of force,” one man calls it, that caught the Department of Corrections off guard. The strike leaders discovered they were more alike than they had ever realized, and that their strength would come from working together toward a common goal.

Similarly, when the 2013 strikes were called, many of the inmates were not in the kind of prison that the original Pelican Bay organizers lived in. Yet they recognized the power of a broad action. Unincarcerated people noticed and took part as well. “The Strike” shows news footage of marching protesters demanding change.

Change happens slowly. There’s no central source of information about the number of U.S. prisoners held in solitary confinement, but it’s far more than most of us realize. The striking prisoners had an effect, but they didn’t change the entire system. Yet what “The Strike” makes clear is that their collective action didn’t just make an incremental step toward better conditions for prisoners. In the end, it changed the way they saw the world, too.