A "storage container" is a defined, specific location in high-volume storage. Some people may call them "chests" or wooden "boxes" or "crates".
A storage container can hold Letters, Bullets, and other goods. It can be opened by interacting with the container and then the loot inside will pop out. You can not. put your own items back inside theses chests.
Storage containers can be found at every camp/biome. There are usually two of these chests for each location. The only exception to this is in the Alpine Tundra biome where there is only one chest.