Simply put, every single time you summon a skeleton, there's a chance that it'll be a certain subtype. The most confusing stat that you want to manage however is Summon Quality. This is great for some additional covering fire if you're going to rely on just overwhelming your enemies with raw numbers. Curiously, if you raise this stat above four, you'll start summoning artillery units, skeleton archers and mages that can attack enemies safely from afar.
Behind that is Max Skeletons, which shows how many skeletons you can have summoned at any time. The most straightforward of these is Skeleton Level, which determines your skeletons' health and attack damage. The Necromancer doesn't rely on straightforward stats like attack damage or counterattack chance, but has its own stats dedicated to summoning their skeletal servants. You build this guy right, he's a one-man army. If they tear through your summons too quickly, or if one monster decides to just beat you up, you'll be completely helpless. Because of this, you need to be wary of going into fights with large hordes of enemies.
It doesn't matter how strong your summons are if the summoner is taken out after all. This makes the Necromancer a glass cannon of sorts.