The final scenes in "Avengers: Age of Ultron" concern a nefarious plan that, as usual, threatens to wipe out all of humanity. The fate of the world usually hangs in the balance with these superhero movies, so it's hard to raise the stakes each time, but director Whedon and his team deliver a doozy. It's too much of a spoiler to describe specifically, so let's just say it concerns what's sometimes referred to as an Extinction Level Event (ELE).
ELEs are events that cause massive loss of life on Earth, including the extinction of entire species of plants, animals and marine life. Researchers believe that catastrophic extinctions, as opposed to gradual extinctions, have been triggered many times in Earth's past by pesky occurrences such as asteroid collisions, massive volcanic eruptions, and even nearby supernovas or gamma ray bursts. The dinosaurs are believed to have been wiped out by an ELE. Probably the most enduring theory, in popular culture anyway, is that an asteroid or comet impacted the Earth about 65 million years ago.
Without disclosing too much about the events in the movie, let's just say this: According to scientific conjecture, if a mile-wide chunk of flying real estate were to impact the Earth, the resulting dust and debris would likely clog up the atmosphere enough to cause an extinction-level event. So, you know, keep that in mind.