Troops require food. Even if you win a “mechanics battalion” on the wheel of fortune, you won’t want to “apply” it until you know you can feed the troops! Many newbies have turned in their mechanics battalion only to find they suddenly were starving because they didn’t have enough food production to keep them. (Or food stored from looting/buying on the market.)
The amount of food you produce will show as either a green number or a red number. Red means you are using more food to feed your troops than you are currently making. This number will change if your troops are marching to war. When they march to war they bring food with them so your food production may look like it’s a positive (green) number. However if you look at the number when your troops are back you might be generating less food than you need (red.)
I don’t worry about food too much as it is easy (and cheap) to purchase on the market.