A Stealth Suite makes a warship much harder to spot on enemy sensors.
Stealth suites do NOT make ships invisible - they can still be seen clearly by the naked eye (although in space combat, by the time an enemy ship is close enough to see with the naked eye, you've already been well within the range of their weapons for a long, long time).
Rather, stealth suites sense the unavoidable electronic emissions from a ship and send out an inverted-phase mirror image of the same signal, thus canceling it out and making the ship much more difficult for the enemy to detect. This device can compensate for stray electromagnetic noise from the internal electronics and some of the emissions from the engine systems, at least as long as the exhausts from ion drives and maneuvering thrusters are not pointed at the enemy. While it cannot totally mask such signals, it can greatly reduce them and thus allow a ship to move in much closer before being detected than otherwise possible. Infrared blackbody radiation is greatly reduced by pumping cooling fluid under the skin of the ship, dumping the ship's waste heat into an internal technomagical heat sink temporarily. This has a limited capacity, so ships cannot remain cloaked for more than a few days at the absolute maximum.
Engaging a stealth suite is informally known as going into "stealth mode". While in stealth mode, a cloaked ship can only use passive sensors. Using active sensors (like sending out a radar or lidar "ping") will instantly alert the enemy to the cloaked ship's presence and location. Likewise, the instant a cloaked ship fires a weapon, it's location can quickly be determined. Gravity Drives cannot be used for propulsion, shielding, or to form jump points for entering hyperspace, as gravity waves cannot be fully cancelled out without interfering with the operation of the device in question.
Stealth suites are sometimes referred to, incorrectly, as "cloaking devices".