If the amount of Food on the Town screen is marked in red, it means that Food consumption exceeds production. You can view information about Food production by opening the Statistics tab in the Palace and selecting the Economic section.

If the amount of Food decreases to zero, don't worry — nothing will happen to your troops. However, you won't be able to train new warriors or initiate other processes that require Food.

You can resolve the issue in two ways:

1. Increase the production of Food in your Town. There are different ways to do so: build more Farms, upgrade existing ones, learn Knowledge, upgrade Skills, use boosts, etc.
2. Dismiss troops you don't need in order to decrease resource consumption. You can do so in the Shrine of Odin: open the "In the Town" tab, select the type and number of warriors, and hit the "Dismiss" button.