An anti-materiel rifle in fact does what it says on the tin. It is a man-portable rifled firearm intended not (primarily) for shooting at personnel, but for shooting at things - materiel. There are a few reasons you might want to do this. Back in the First World War, when these first made their appearance, early tanks and other battlefield vehicles were relatively fragile and could be disabled (if not destroyed) by a good shot with a large, high-energy cartridge. Usually, the anti-materiel rifle shares a round with a large-caliber automatic weapon. The United States produces a number of these weapons which use the classic M2 Browning machine gun .50 caliber cartridge. Several Eastern Bloc variants use the 12.7mmx108 Russian machine gun cartridge, which is roughly the same size as the NATO .50.
In addition to small or lightly-armored vehicles, these are useful for destroying unexploded ordnance from a distance, or for destroying signals, antennas, and even light weapons like mounted machine guns from far away. Although they can indeed be used for shooting at people, they're large enough that it's difficult to track quickly or unpredictably-moving targets with them - and they can really only be fired from a prone position.
The term has made the jump over to video games, of course. The game Fallout features a .50 caliber named Anti-Materiel Rifle which, indeed, does a ridiculous amount of damage but is extremely heavy.