I think Trasher meant this resource like a population cap, like in Warcraft.
You have a max pop cap of 50, so if you build to many units (swordsman 1 food, dragon 5 food) and reach the max pop cap you can't build more units. Hope u get it.
PS: In Glest you can build limitless units, just have to build more farms,
cause there is no population cap.