A metal clothesline for food. Some people call it a "Bear Cable". (As if bears even knew about TV.)
A permanent wire or cable hung between two trees, usually at least 10 feet high (3 m).
Other wires descend from this, sometimes hanging from pulleys, so food bags can be raised to and lowered from the safe height of the bear wire.
It's all metal so critters (mice, skunks, raccoons, chipmunks, squirrels, bears, and so on) can't gnaw through it easily.
Easily is the key word, since one or more of them might succeed in gnawing through it if given enough time, but generally, for any animal that hungry, it's easier to eat you instead of going through all that painful gnawing.
Hey, it's all about hunger in the back country.
In some places the "bear wire" might be a metal contraption something like a tall coat rack with hooks at the top, which requires you to raise your food up to it by using a (provided) metal pole, which is yet another reason never to go backpacking.