When the Earth happens to find itself in the crosshairs of a meteor, people tend to panic at the thought that it's the end of the world. On the contrary, most meteors explode and break up before ever reaching the ground, and you have the Earth's atmosphere to thank for that.
Meteors move at high rates of speed through space, where there's virtually no friction. As soon as they have to fight friction in the Earth's atmosphere at those speeds, the meteor heats up, and its structural integrity has a hard time overcoming the excess drag.
When presented with tremendous amounts of drag, the side of the meteor facing the friction moves more slowly than the side that isn't combating these forces. Consequently, the meteor compresses, becomes unstable, and collapses.
A meteor's composition and size each have a lot to do with its probability of breaking up. Meteors rich in metal have a higher resistance to compression, while chondrite-rich stonier ones are more susceptible to shattering under these forces.
Luckily for anyone with meteor phobias, most shatter before touching the ground. It's incredibly uncommon that a giant meteor survives the fall and contacts the ground.