When one of your capital ships dies and the Progenitor is in the gravity well, the Progenitor will "store" the destroyed Capital Ship's level (let's say 7) if it has Resurrection.
This is a common misconception that shows just how little people understand this ability (not to pick on you Mr. Haze).
The mothership does not have to be in the gravity well. If you had a level 10 Radiance battleship that gets destroyed anywhere in the galaxy, and you then build another Radiance at your home world with a level six Mothership orbiting it, if you use Resurrection on the Radiance after a moment it will be promoted to level 10. If you had a level 6 Halcyon then die, if you rebuilt a new one and used resurrection on it, the new Halcyon would be level 6. If you built a Rapture but haven't lost one, it will do nothing. If you built another Radiance before the level 10 one was destroyed, it would also do nothing.
I'm a little bit fuzzier on whether the mothership needs the resurrection ability before it start keeping track of if you lose multiple say Halcyons and then try to use resurrection on multiple rebuilt ones, but its a powerful ability for Advent late game that most people don't use to their advantage.