The interphase cloak was an experimental Romulan device that shifted a ship out of phase with normal matter, making it undetectable and able to pass through objects.
The Enterprise-D encountered the technology while assisting a disabled Romulan science vessel whose engines had been damaged by an interphase generator experiment. During the rescue, a transporter accident left Geordi La Forge and Ro Laren out of phase, invisible to their crewmates and able to walk through walls.
La Forge and Ro discovered that a Romulan plan threatened the Enterprise and found a way to signal their presence. Data eventually used anyon emissions to restore them to normal phase.