Primaris Ancients are Primaris Marines dubbed honorable enough to have the privilege of carrying the Standard for a company or full Chapter of Marines. Identifiable by their white helmets (although that could change depending on the Chapter), they are in no way related to another dude-clad-in-power-armor-wearing-a-white-helmet-that-also-comes-with-the-same-name-from-a-certain-WH40K-game.
The Primaris Ancient functions pretty much the same as regular old Adeptus Astartes Standard Bearers with the exception of the equipment and armor they are carrying.
In case you were wondering, "Ancient" is a old corruption of the rank of "Ensign" which was a term for an army's flagbearer in the Middle Ages, so it has nothing to do with how old the Marine in question actually is, that said, since you have to earn enough honor to hold the standard, these guys are likely somewhat ancient.
The Primaris Ancient is often confused with the Primaris Apothecary as they both seem to offer some buffs or special support abilities to their Primaris comrades. Usually the confusion stems from the fact that from a newfag's perspective, both of these units have a similar ability to recoup some of the value from slain marines, and both of them seem very worthwhile when teaming up with the more expensive Primaris marine choices.
The key difference to differentiate between the two however (Other then the obvious differing appearance of the marines), is that the Ancient allows a mortally slain allied unit to fight once before dying (Think of it as a last stand or the Belisarian Furnace working in action), whilst the Apothecary tells him to suck it up, stop being a pussy and get back in the fight. Both of these marines offer unique playstyles with neither of the two being blatantly superior then the other. So whether you want to have an Ancient or Apothecary is up to you.
Basically, if you want to have a unit that brings back downed units, then choose the Apothecary, if you want to allow your dying unit to offer one last fuck you to the enemy, then choose the Ancient. And if you want that dead unit to come back and do it again next turn, you take them both.
|Forces of the Primaris Marines|
|Command:|| Primaris Ancient - Primaris Apothecary - Primaris Captain |
Primaris Chaplain - Primaris Librarian - Primaris Lieutenant
Vanguard Librarian - Vanguard Lieutenant - Helix Adept
|Troops:|| Aggressor - Eliminator - Hellblaster - Inceptor - Incursor |
Infiltrator - Intercessor - Reiver - Suppressor
|Vehicles:|| Impulsor - Invictor Tactical Warsuit |
Redemptor Dreadnought - Repulsor Tank
|Super Heavies:||Astraeus Super-Heavy Tank|