The Anti-Matter Meteorite was a solid piece of debris that originated in outer space and was composed solely of Antimatter.
Coming from outer space, the Anti-Matter Meteorite had a trajectory to the coast of Miami, and it would have splashed down with an impact strength that would have devoured the city. However, S.H.I.E.L.D. was able to stop the meteorite before the public knew of the potential catastrophe.
The meteorite was later used by Agent Phil Coulson as an example of how keeping dangerous events quiet and contained is very important to avoid spreading panic amongst the population when Skye thought about warning the people who lived around Llactapata in Peru of the potential danger of the 0-8-4 discovered there.