(covers information from several alternate timelines)
A spatial rift is a physical opening in the spacetime continuum, which allows for passage from one point to another.
At the beginning of their journey in 2371, Captain Janeway was hoping to find these along the way to shorten their journey home from the Delta Quadrant. (VOY: "Caretaker") Q, Junior had the knowledge of how to open a type of rift, known as a spatial flexure, with nothing more than a standard deflector array. (VOY: "Q2")
In 2372, after the USS Voyager was duplicated by an encounter with a subspace divergence field, the crew discovered a spatial rift on deck 15, linking the two Voyagers, which existed at the same physical location, but slightly out of phase with each other. When Kes accidentally traveled through the rift, she lost consciousness due to the shock of spatial transition. The crew was eventually able to rig a portable phase discriminator to protect those traveling through from the transition, allowing both crews to interact with each other. (VOY: "Deadlock")
In 2377, Voyager encountered a spatial rift emitting high levels of neutrinos and chronitons. The rift produced a powerful chronokinetic surge which interacted with Voyager's warp core, shattering the spacetime continuum aboard the ship, fracturing it in to thirty-seven different time frames. Commander Chakotay, working with Captain Janeway's counterpart from the year 2371, was able to distribute a chroniton-infused serum throughout the ship, utilizing Voyager's bio-neural circuitry. When the process was complete, the warp core was recalibrated to generate a chroniton field, bringing Voyager back into temporal sync only seconds before the chronokinetic surge struck the ship. With only moments, Chakotay was able to reroute power to the main deflector and reset its polarity, using it as a lightning rod to absorb the surge, averting the accident. (VOY: "Shattered")