Negative idle labor indicates that your production requires more labor than your current population provides and is not fully functional due to the shortage of population. You cannot train troops until you get the idle labor back positive.

Your population is determined by your max population and morale. To have more population, you can:

 -Increase your max population by building/upgrading civilian houses.
 -Increase your morale by dropping you tax rate.
 - Summon more people from your city hall at the cost of food.
 -Use Immigration Bill and/or Immigration Rules to instantly increase population.

**Note: Hazards happen when idle labor exceeds 50% of max population.