The mental state of your trek members. Drop to zero and more and more catastrophic events will begin to occur.

Sanity isn't so much a "sanity" meter as it is sort of a combination of your party's supplies and morale. Traveling at 0 sanity is very dangerous, and can result in your party members dying, running away, disappearing, eating each other, etc. If you are the only one left in your party, traveling at 0 sanity could mean your death, although the Lone Survivor perk will prolong that somewhat.

The most reliable way to restore your sanity is through Chocolate Rations. While not the most abundant or powerful, there are no negatives to it. Mangos are also good with no negative effects from eating, but will decay and disappear from your inventory after a few days, and can't be taken between expeditions. Cooked meat is also a good choice, but it requires you kill the local wildlife, and a Cook (preferably high level). Whiskey will restore more sanity than chocolate or mango, but has a chance to make one of your party members an alcoholic (which isn't the worst ailment to have). Coca Leaves, on the other hand, should be a last resort, as they have a high chance of making your party members go insane, albeit the less harmful ways of coming to suffer on an expedition. Drums can be used to restore your sanity by a little bit, but will make you more likely to be attacked by wildlife for a period of time. Food Cans are cheap and great, but they only raise sanity up to 31-40.

Another way of regaining sanity is to rest. You can rest at a village, but will cost you some standing with the locals. If your standing is low, you'll have to barter to stay the night. If it's low enough, the natives will even refuse. Missions are similar in that you barter to rest there, but having a Missionary lets you rest for free. There are a few locations that let you rest out in the open, and the Camp Site item can be expended to create a resting site on the map. Resting takes time, and can often lead to random events, depending on your location. Food Cans

Protip: Sanity is key! Don't be afraid of wasting a lot of time by resting outside. It's better to come home with little fame than to not come home at all! Also use sanity recovering items only when needed (with the exception of mangos). You don't want to chug a bunch of whiskey, only to realize that the pyramid is right around the corner.