The M-19 meteors were a shower of iron-rich meteors passing close to planet Earth.
It is implied that these meteors passed close to Earth every 50 years or so. The International Space Station was in position to use as an observatory to study them. However, on February 2007, the station suffered damages due to a micro-meteor shower and was at risk of getting out of orbit and missing out the meteor shower. The Fantastic Four were trying to repair the damages, but they were unable to do so without spare parts and the space program director couldn't send a shuttle to aid them. Fortunately their call was heard by Superman, who assisted them and repaired the damages himself.
Several months later, when the meteor shower was in close range, Magneto used Asteroid M to greatly amplify his powers in order to pull the meteors towards Earth's gravity well, in an attempt to wipe out humanity and claim Earth for mutantkind. The attempt was foiled by Superman, who managed to destroy every single meteor with his heat vision before they could cause any damage.