Please help me understand province "supplies"

[Arryn]

Most units in the game eat. (Some don't, and they have the "Need not eat" icon to show that.) Those units which eat comsume supplies equal to their Size -1. So a Jotun giant of size 4 eats 3 units per month, while a human of size 2 eats 1 unit. Avoid piling too many troops in one spot unless you have means to feed them all. Nature mages create surplus food at a rate of 5 units per level of Nature magic path that they have. Various magic items also provide for extra food.

Rather than repeat for the Nth time all this info, I recommend that you do a forum SEARCH on the topics "supply" or "supplies", in the message body. There have been several excellent threads that have covered this in the past few months.