A humidor is a box designed to keep humidity and temperature steady. The usual lining is Spanish cedar, which absorbs and releases moisture once you have seasoned it. A hygrometer measures the relative humidity, and a humidifier, humidity packs, or beads keep it at the level you want.
There is no single correct number. Manufacturers, shops, and collectors commonly recommend a range from the mid-60s to the low 70s percent RH. The older American rule was “70/70” (70 percent humidity at 70 °F). Some Cuban storage advice is slightly drier and cooler. Choose a steady level within that range and keep it there instead of constantly adjusting. Details are in the humidor article.
Smoke Rings sells humidors in its shop.