That is an excellent question, true believers. The words were first uttered in a conversation between Captain America and Falcon during the post-credits scene during Ant-Man.
So what they heck are they, and why have they put a barrier between Steve and Tony? Between the release of some of the artwork that goes along with the Marvel Phase Two massive box set and the Captain America: Civil War trailer, we finally have the start of some answers.
The Sokovia Accords is the MCU version of the Superhero Registration Act
In this case, Sokovia refers to the fictional European city that was made airborne, threatening global destruction during Age of Ultron. A few years ago, Marvel comics pitted Iron Man vs. Captain America after a horrific incident that turned the citizens of Marvel against its biggest superheroes. In the comics, the Registration Act was all about revealing your secret identity in order to continue super-hero-ing in the United States.
But what does revealing your secret identity mean to the heroes of the Marvel Cinematic Universe? Iron Man revealed his identity during a press conference, Captain America has an entire Smithsonian exhibit devoted to him, and apparently people already know that Bruce Banner is the Hulk. Back in the '40s and '70s, the government (or S.H.I.E.L.D.) controlled the only super-humans around, Captain America and Ant-Man, and Ant-Man didn't have a movie or USO tour based around him, he was a secret, classified weapon. So, in this case, it's not about secret identities. Take a look at the Civil War Trailer, nothing too secret about any of them!
The trailer (and Thunderbolt Ross) makes it clear that the Accords are all about the government coming down on superheroes who have acted with impunity. With no regulation whatsoever. To paraphrase the Russo Brothers, just because the Avengers are first responders to a super-human crisis, doesn't mean they have the right. I can see where they're coming from. Look what general destruction these heroes have wrought. Even if they don't mean it or are protecting innocent people. With no regulatory committee telling them what to do or where to go.
Here are some examples of that complete and utter destruction...
Iron Man: Huge battle with Iron Monger on the freeway, followed by a massive explosion at Stark Industries.
Incredible Hulk: Huge battle with the army twice: during the destruction of a successful soft drink factory in Brazil, and on American soil at Culver University. When Blonksy became the Abomination, he and Hulk fought, "breaking" Harlem.
Iron Man 2: A battle between Whiplash and Iron Man in Monaco, at the Grand Prix, and another one involving armed drones during the Stark Expo, that resulted in massive explosions in Flushing Meadows, New York.
Thor: The near-destruction of a small town in New Mexico.
Avengers: Aliens invaded New York, after opening a portal right above Stark Tower. This changed the MCU world from that point on.
Iron Man 3: Multiple explosions caused by experimental enhanced soldiers, a.k.a. the Mandarin campaign against the United States; the kidnapping of the President of the United States/destruction of Air Force One; and the destruction of an oil freighter.
Thor: Dark World: A dimensional anomaly in the middle of London, England and the crash landing/invasion of another alien force in Greenwich. Thor fought them single-handed, causing destruction in London and other global locations.
Captain America: Winter Soldier: First off, there's the fact that HYDRA had an enhanced individual--the Winter Soldier--performing assassinations for the past 70 years. Then there is the revelation that S.H.I.E.L.D. has been taken over by HYDRA. A pitched battle in DC between Captain America and the Winter Soldier. And of course, Helicarriers coming down all over the Potomac River.
Avengers: Age of Ultron: Hulk attack in Wakanda, sentient robots attacking all over the world, and of course, SOKOVIA.
Agents of SHIELD: You name it, they experienced it: explosions caused by ghosts, Beserker Aryans, HYDRA.
Ant-Man: The destruction of the Pym Technologies building (in the form of a tank and explosions) and shrinking technology in the hands of HYDRA.
Daredevil / Jessica Jones: Vigilantes and super-powered individuals in Hell's Kitchen, New York.
- Captain America: Civil War: Just some punk kid web-slinging around New York.
As the Winter Soldier, Bucky Barnes did a lot of bad stuff for HYDRA. He's apparently being framed for the explosion at (what appears to be) the UN. Plus, he's an unregistered enhanced super-human. He would be the perfect poster boy for Ross and the Accords to go after for dangerous individuals with powers.
Now this brings us to the actual Sokovia Accords. This image is from the new Marvel Phase 2 Box set...
I... shall no longer operate freely or or unregulated.. acting only when and if the panel deems it appropriate and/or necessary.
Essentially, the Avengers can't act as a global super-human police force unless a committee says so. No more acting on your own. You promised. And signed a federally-binding UN document.
Check out this shot from the new Civil War trailer.
It actually says on the front cover the 'Framework for the Registration and Deployment of Enhanced Individuals' with a UN logo, making it an official global document. Like HUAC of the 1950s, the Avengers are forced to sign this document or risk going to jail.