The immortal is indeed a SC2 unit.
Likr all protoss units, some of their durability comes from shields. Some background mechanics info: Shields recharge automatically out of battle, while HP does not. Most protoss units (as opposed to structures) have about 33 to 50% of their durability into shields.
The immortal though has a special mechanic such that for any normal source of damage, that "bullet" will not do any more than N damage (currently, N is at 10), provided their shields was >0 a at the time of impact. This is applied at the last step* of the damage calculation, so it even helps to mitigate bonus multipliers. It also itself has a decently strong but low rof attack.
(Note that I said normal damage. Spell damage or damage that uses non-standard damage calculation can bypass it.)
This makes it great against the slow heavy hitters, but struggles against the high rof or easily "spammable in high number" units.
I must admit, this is a very clever idea from Blizzard to help give an answer to the "heavy hitting dominating all" phenomenon that can easily crop up in games of this nature.
*OK, third to last if you factor in hallucination damage taken penalty and guardian shields, but the point is it comes after standard bonuses of the attacker.