The safehouse is exactly what it sounds like it would be. It's your own personal safehouse. It's only a singleplayer mission at this point, where you can enter a laundry and take a secret stairway down to your own personal armory. Inside, you'll find a firing range, rooms to test out your drills, C4 charges, ECM jammers and various other equipment, trophy-walls for your weapons and masks, and - easily the most satisfying - the vault.
Check out the video above for a nice demonstration. Payday 2 is set for release on August 27th, and my preview is already up.