The unkeep is the food required for your army. As you train more troops, the unkeep will go up. That’s why, you need to keep a close eye on your food production and on the fact that the production should be higher than the unkeep, even by a small 10.000 / hour.
You can get more food from plundering other cities or by upgrading your farms (their upgrade require less time than lumber, stone, ore mines).
If you have like 50.000 might and your troops get killed, your unkeep should go down to zero and it won’t cost anything.