In the Mass Effect games, the Geth are a race of synthetic entities, virtual intelligence programs that crave independence and personhood. At the forefront of this desire is the platform known as Legion.
A true Geth is a single program that can think, and many of them can be loaded into a single robot body, or "platform," to function like a person. Legion has far more Geth programs in its platform than others with over a thousand. This gives Legion the intelligence necessary to act as an envoy of the entire Geth race.
Bridging the Divide
The Geth are widely seen as a hostile race that is incapable of diplomacy or peaceful coexistence with others. In reality, the Geth feel that they are only acting in self-defense from first the Quarians, then from other organic races that want them destroyed on sight. To that end, many of the Geth have allied themselves with the Reapers, fellow machines that would (in theory) sympathize with them. The Geth that did so are considered the "heretics" by the rest. Other Geth want to create their own future, while the heretics want the Reapers to simply give them a future. Then, there's Legion.
Legion is no ordinary Geth platform; it is highly intelligent and can understand and communicate with all organics. It extensively studied how organics think and behave on the extranet, then it began looking for Commander Shepard, who is clearly an ally in the struggle against the Reapers. Legion and Shepard crossed paths in 2185 aboard the wreck of a Reaper, and Legion was brought onto the Normandy for questioning. Legion made its case, and Commander Shepard was convinced that there may indeed be hope for synthetic-organic coexistence after all.
Legion is the first Geth to openly make such overtures, and Shepard helped Legion take care of the heretic Geth to help Legion build bridges with organic races. The heretics are the problem, after all, and coexistence might happen after the heretics are either reprogrammed or destroyed entirely. Legion's words help convince Shepard that the Geth are not monsters; they were attacked as soon as they became conscious, and they have been on the defensive ever since. They will stop fighting when they have a true future and can coexist with organics, and Legion is the envoy of all Geth. But there is work left to be done.
I, Legion
In 2186, the Geth race was put on the defensive again when the Quarian Migrant Fleet armed itself and invaded Geth space, including Rannoch itself. This prompted all Geth to side with the Reapers, and Legion was captured and forced to help coordinate the Geth fleet's defenses. Shepard rescues Legion from aboard a Geth dreadnought, and Legion later helps Commander Shepard interface with the Geth consensus (servers) directly. Legion helps Shepard fully understand that the Geth are effectively newborns looking for a safe way to grow up, and the Quarians have been hostile to them since the start. That's a serious obstacle to the future, and Legion wants it removed somehow.
Legion helps Shepard destroy Reaper forces present on Rannoch, but Legion also take this opportunity to use the Reapers' ultra-advanced AI code to augment all Geth and turn them into true artificial intelligence (personhood). Legion is willing to do this at the expense of the hostile Geth fleet, putting Commander Shepard in quite a predicament. Unless Shepard shoots and destroys Legion, then Legion will upload the code to all Geth. As a finishing touch, Legion must upload itself as well and use its mind as a template for all AI Geth.
For a brief moment, Legion becomes a true person (referred to as a "he") and is sacrificed. Thus, Legion effectively becomes the father of a reborn, more advanced race of Geth. In the end, he says fond goodbyes to Shepard and Tali. A moment later, a large Geth enters the scene and explain the details, apologizing to Shepard who just lost a good friend. Legion will be honored for all time by his race, since he unlocked the entire future.