Food is required to maintain your Units. If your Resource indicator on the top of the screen shows a negative balance next to your Food supply, it means that your Units are consuming more Food than your Farms can produce. You can increase your Food production rate by upgrading your Farms or purchasing a 3-day Food Production Boost at the Black Market.
If you don't have enough Food to support new Units, you will see a warning message in your Unit creation interface.
In order to see your Food Production and Consumption rates, tap on your avatar in the upper left corner of the game screen. Scroll down until you see the Resources section and then check the Food Production and Food Consumption rates.