Supplies are what supports most units, preventing Starvation and displeasure. They are presumably delicious.

Armies in Dominions don't carry even a month's worth of Supplies (one turn's worth) as they move. Once they're established in a province, they live off the land there. This is reflected in the Supply count for that province.

Outside forts, Supplies are in the same category as Recruitment Points and Resources, in that there is no carryover between months; while Recruitment Points represent the province's levy capacity, and while Resources represent the province's industrial capacity, Supplies represent the province's capacity to feed armies.

Inside forts, Supplies are replenished by the lands outside, but that is no longer the case when under a Siege. Under a Siege, the Supplies that aren't eaten gradually perish. Take the fort's maximum Supply capacity, then divide that number by how many months/turns have passed since the Siege began; that's the maximum number of usable Supplies remaining, after the fort defenders have their meals.

Most Size 3 units consume 1 unit of Supplies each turn. This does not change in good seasons, nor in bad ones.

Starvation occurs when the needs of the belly aren't met. Not everyone starves when there is a deficit in Supply; meals are provided to most (or, in terrible situations, some) and are not provided to others. Generally, commanders are fed first, and animals are fed last.

Provinces with a higher population generally have more Supplies. They have more farmers, after all.

Supply availability also changes with the seasons, due to how they tip the Temperature Scales. Every nation has a preferred Temperature (such as 0 or 1) that they are used to farming in, and a different temperature from that reduces Supplies.

The Growth and Death Scales also change Supply availability.