Lifeblood-Gems are currently elusive and rare; they are very similar to a soul gem, but instead of powering up a charge (for a weapon) or reviving a downed ally, this unique gem holds the lifeblood force of a NPC and allows a vampire to feed by simply using the gem like any other consumable in his/her bag.
Lifeblood-Gems take away the redundant need to feed while doing lengthy quests, prolonged group dungeons, and/ or if running in an area that has a low population of NPCs to feed upon. (All vampires can relate to these scenarios and truly understand the negative impacts of being in Stage 2, 3, and 4, because they are in situations where feeding is completely unattainable.)
Traditionally, a vampire player crouches behind a (cattle/drinkable) NPC and is prompted to press “X” to feed, but when they attained an empty Lifeblood-Gem on their person, they are able to fill the gem simultaneously while feeding to be used at a later date/time.
An additional variant to this, from my understanding, is the transformation of the consumable known as “Bloodforth”. This consumable is crafted with provisioning or alchemy and is equal to the Lifeblood-Gem, thus permitting a vampire to feed in certain situations when proper feeding is unattainable; these recipes and gems are for the most part unattainable in ESO.