Population is counter that shows how many units you can have and how much more you can have. If you are training a unit but that population limit reached its max, the progress bar will pause and you can't train any more (if you have any researches behind your paused unit, you can always tap the unit's icon to cancel it). If you think that 24 units are too little, you can select your HQ and choose the research button then tap "Population Limit+ I" to increase the popultion to 34. After the first research/upgrade, you can do a second research (Population Limit+ II) which will increase the limit to 40. Sadly, the second upgrade is the last so you can't increase it anymore.

Tip: You should only train 2 units per type to save some space for the population limit, i.e. 2 Gunners, 2 Strikers, 2 Slammers and 2 Prowlers for the Garrison. Only train more when you think that you still have enough room for and extra unit or two.