Real quick question...what does HD mean?
I've looked through the core books, and I can't find it.
Haha, and I'm pretty sure it is commonly understood...
It means each of those monsters has 2d4 (a 4-sided die twice) in hit points (or 5 on average). No single typical example of that monster can have more than 8 hit points without extenuating circumstances, such as a high constitution score, the toughness feat or additional class levels.
The sum is the total HD you can affect. If you rolled a total of 5, then you could affect a 3rd level fighter and a 2nd level Rogue, for example. Or you could have affected 5 orcs (each have 1 HD). Etc.