The Federation holoship was a cloaked vessel hidden in a lake on the Ba'ku planet in 2375, containing a holographic copy of the village so its people could be removed without knowing.
The holoship appears in Star Trek: Insurrection. Picard, Data and the Ba'ku woman Anij find it submerged in the lake near the village, and wade inside to discover a huge holodeck running a perfect recreation of the valley. Data had stumbled on it earlier, which is why he was shot and began to malfunction.
The plan, agreed between Admiral Dougherty and the Son'a, was to beam the six hundred Ba'ku aboard while they slept and carry them away to another world, leaving the planet's rejuvenating rings free to be harvested.
Picard later turns the ship against its makers. He has Ru'afo and his bridge crew transported on to it, into a simulation of their own command deck, so that they believe they are still in control while the collector is shut down.