Wormholes are a type of spatial phenomena in which a Singularity such as a Black Hole links up with another black hole somewhere else in the universe, presumably through extra dimensions, creating a shortcut through space and time. Wormholes are frequently used as a means of Faster than light travel and occasionally as Time Travel.
Stable wormholes are rare in nature, though sometimes incredibly advanced civilizations have been known to create them. Unstable wormholes are moving rapidly through spacetime, on both ends. This often leads to hazards to travelers moving through the wormhole.
The Bajoran Wormhole in the Bajor System of the Alpha Quadrant in the Milky Way was one example of a rare stable wormhole. It connected the Alpha Quadrant to the Gamma Quadrant. Additionally, a sentient species lived in the wormhole and may be responsible for the wormhole's stability. (Star Trek franchise: "Star Trek: Deep Space Nine")
The Stargates create stable wormholes on a regular basis from one gate to another. Matter can move through these wormholes to be compressed into a matter stream and later reassembled. The Ancients were responsible for inventing the Stargates. (Stargate franchise: "Stargate SG-1")