They accomplished what security guards and military pilots and government officials could not—they impeded the terrorists, giving their lives and allowing hundreds or thousands of others to live.
On September 11, 2001, a group of forty people who didn't know each other banded together to save themselves and others from terrorists. United Airlines Flight 93 was supposed to be a routine flight from Newark, New Jersey to San Francisco. However, it was not a routine flight at all, because four hijackers were on board with the intention to crash the plane into what is now believed to be the US Capital building in Washington DC. The people on Flight 93 were ordinary people, going about their ordinary lives when the terrible events of 9/11 happened. But because the flight took off about forty minutes late, and because many passengers called loved ones, they knew about the other crashes/attacks so they decided to do something--fight back.
We do know that air traffic control got a mayday message from the cockpit as the hijacking began. It is believed that the co-pilot, LeRoy Homer, was shouting: "Mayday! Mayday! Get out of here! Get out of here!" Homer, along with pilot Jason Dahl, were most likely killed right away by the hijackers in order to gain control of the cockpit so they could fly the plane back to its target. With the pilots dead, it would be easier to control the passengers...or so they thought. The first class flight attendant was also killed in the melee, along with first class passenger Mickey Rothenberg (who probably tried to stand up to the hijackers and talk them down and was killed in a show of force).
Many of the passenger's families got phone calls from their loved ones, including Tom Burnett, Jeremy Glick, Mark Bingham and Todd Beamer. They, along with other passengers, were forming a plan of attack, they couldn't just sit by and be killed. We have Todd Beamer's famous quote: "Are you guys ready? Okay, let's roll!" Based on the plane's recorders, we know that the passengers did breach the cockpit by battering the drink cart into the door. Two men on the flight, Donald Greene and Andy Garcia, did have knowledge of planes. Greene knew how to fly and Garcia had been an air traffic controller in the military, but it's unclear if part of the plan was to install these two after removing the hijackers (but they were probably thinking about doing just that).
Anyway, these were people who scarified themselves to save others. Like I said, they didn't know each other, but decided to take action and ended up saving the lives of countless others in the nation's capital.