I am GM and trying to setup a Reign of Winter story for my players. I came across the small ice elemental's and they have hp that reads hp13 (2d10+2). I am still new to Pathfinder and wondering: what does that have to do with hp? Why is a dice roll required when doing a creature hp check? I thought those rolls were mainly used for damage.
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The dice roll is to determine how many HP a particular creature has, since it makes sense that they're not all identical. However, when it appears after a number of HP that means it has already been rolled for you, and the roll is just there for informational value.
In this case, the ice elemental has 13 hit points (which is the average of 2d10+2). For just this one elemental, "13hp" is all you need and you can ignore the roll.
The roll is there so you know how hit points are normally determined for this creature and how many "hit dice" it has, in case you need to know. For example, if you wanted to make the elemental harder you could give it more hit points, up to the maximum number possible for the roll (22hp); or weaker by giving it less, down to its minimum of 4hp. In the same way, if you wanted to have more of this kind of elemental appear, you know how to randomly generate a hit point amount, or if you want to just choose its hit points you know what the minimum, maximum, and average is. And if an effect depends on how many hit dice the elemental has (a spell targeting it, for example), then you know that it has 2 hit dice.
That line is telling you how much HP the elemental has. On average, they have 13 hit points. If you want to roll randomly for the HP of the monster, you can instead roll 2d10+2, and use that as it's hit points instead. The reason that 13 HP is mentioned is that the average result for 2d10+2 is 13. There should be a similar line for the hit points of every monster.
Like players, monsters can have their number of hit points determined by a die roll, with one die rolled for each Hit Die the monster has. Those elementals have 2 hit dice, and a Constitution bonus of +1. For elementals, the normal Hit Dice are d10s, like Fighters. They add their Con bonus to each Hit Die, which is where the +2 comes from.
In order to fully understand this, you need to understand the concept of "hit dice".
Player characters gain hit points by gaining levels. For example, a Fighter gets 1d10 new hit points, plus his Constitution modifier, with each new level.
Monsters from bestiary entries usually do not have levels. Your ice elemental, for example, does not have a class. Yet monsters need to have hit points.
As a result, they have "racial hit dice". This is roughly equivalent to a class, but only for monsters. For example, if you look up the description of outsiders in the appendix of the bestiary, you'll find that they get 1d10 hit points per hit die, plus their CON modifier.
So let's look at your Ice Elemental. Its line says:
It has two racial hit dice (2d10). The exact number of hit dice the creature has matters for purposes of some spells, such as Sleep (which only affects a total of four hit dice worth of creatures per casting). Therefore, the number of hit dice are listed so you'll know for things like that.
The number after the hit dice, 2, is the number of hit dice that the Ice Elemental gets from its Constitution. It must have a CON of either 12 or 13, and the resulting +1 modifier gives it +2 hit points for its 2 hit dice.
The "hp 13" is the hitpoint total you should use. Monsters are assumed to have rolled an exactly average number of hit points at each new level. In this case, that's 5.5 hit points * 2 hit dice = 11, plus 2 from Constitution, yielding the listed 13 total hit points.
Because this is often puzzling at first glance, hit die averages always end in .5. That's because you cannot roll a zero on a die. Thus: (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10) / 10 = 5.5.