A thrall is a kind of serf (in [medieval] feudal systems, a serf pledged his labor to a lord in exchange for protection and/or a piece of land to toil).
A fledgling is someone young, new or inexperienced.
My guess is that mortals agree to become servants to vampires in exchange for the promise of being turned and acquiring vampiric powers and priviledges.
On a side note, vampire deaths look much better now. A slain vampire disintergrates into a pile of ash, which you can scoop up, and you might also find gold and other loot if you search the ashes.
This makes much more sense than in Oblivion, where you could take vampire ashes out of an intact corpse.

Those apparently are Vampire fledglings
But anyway, maybe I'm wrong, I'm sure Bethesda could have modelled more than one scary looking old woman
Good news for variety though 



