Not temp HP. Normal HP.
So, for example, a 4th level barbarian with a 16 Constitution normally, with 43 HP normally, when raging will have 55 hp.
Suppose he takes 10 damage while raging (45 HP in rage) - post-rage, he has 33 HP. If he had taken 44 damage while raging (11 HP in rage), at the end of his rage, he would drop to -1 HP and dying.
On it's own, yes, that's rather weak.
However, do note that the barbarian also gains +4 Strength while raging as well - which, with a 2-handed weapon, equates into +2 attack, and +3 damage, and he can power attack for a lower penalty.
He also gets a +2 morale bonus on his Will saves for the duration of his rage.