The Mirror Universe is a separate galaxy with around 50 systems, ruled by the Terran Empire. You cannot travel freely in and out. Enter through an Interphasic Entry Rift at Mirror Alpha Centauri, Mirror Wolf or Mirror Sirius. Each ship costs 300 Mirror Vouchers. You can buy more in the Store. Alliance mates can come too, but each player pays their own Vouchers.
A Voucher gives 20 minutes of base duration. When time runs out, your ships are auto-extracted and keep everything they gained. To leave early, you cannot recall a ship. Find an Extract Rift and use it.
If your ship is destroyed, everything on it is gone, including Trellium. Other players can hit you anywhere in Mirror Space. Know where your nearest Extract Rift is before you start fighting. Keep your Apex Barrier active.
Going deeper needs Rift Keys of Common, Uncommon or Premium rarity. They come from the Mirror Refinery. You cannot use Discovery jumping or Gorn wormholes to enter. Trellium exists only here, in raw types A and D. Mine it or take it from Terran hostiles. Refine it into Liquid Trellium in the Mirror Refinery, which unlocks at Ops 40. Liquid Trellium-D upgrades Mirror Universe officers like Picard and Data.