New Spanish cedar is very dry and absorbs moisture readily. Seasoning means raising the moisture content of the wood so the box holds its relative humidity instead of drawing moisture out of the first cigars you put inside. The usual method is to wipe the wood with distilled water or place a container of distilled water inside (do not use tap water, and do not soak the interior, which can warp the joints), close the box, wait several days, check the hygrometer, and repeat until the reading stays in your target range.
There are several methods: a sponge, a shot glass of water, or humidity packs made for seasoning. All of them require patience. If you put cigars into a dry box, the wrappers can crack.
After seasoning, the humidor still needs a source of moisture. Seasoning is done once at the start; it does not replace a humidifier. Full instructions: how to season a humidor. Storage overview: topics · humidor.