Hold your mouse over the people on the bottom right of your screen (when on the supply tab).
Your population limit is determined by your cottage capacity.
Your current population is a percent of the limit equal to your happiness.
Your labor force is the number of your population that you are currently employing in your buildings and fields. Your idle population is the number of population available to work in buildings or be trained as soldiers.
Idle population trained as soldiers will replenish over time (depending on your happiness level).
If you want more population you need to build cottages or lower taxes.