@Book
Well, a slipstream is entered by opening up a portal in space. The portal is pitch-black for the normal eye.
It is used as a means of FTL (Faster than Light) travel, but has been used in one instance as a way to create an explosive, sending pieces of ship to another dimension. In Halo, there are seven dimensions, but only three "space" dimensions affect our universe. Slipspace is creating a portal into the other four dimensions and using it to travel to other locations. The physics there is different and non-shielded craft and sometimes even people tend to simply disappear from existence.
It is pretty much a wormhole, except that it creates many tiny black holes that eventually converge into one, or at least that's what I gathered.
The relation it has with the storyline is that it was the reason John was sent to this universe. Thanks to his luck, John and Cortana were simply transported to this time instead of simply vanishing from existence. It's not supposed to work like this, but a malfunction caused the whole trip to sort of accelerate and after 3 hours, they traveled 519 trillion years into the future, into the time period of this RP. This is known as a Slipspace Anomaly, specifically a Time-Space Rip, and has happened in the Halo universe before, except that it sent people into the past.
If you want to know more about Slipspace and the like, here's some pages. They can explain it better than I can.
http://halo.wikia.com/wiki/Shaw-Fujikawa_Translight_Engine
http://halo.wikia.com/wiki/Slipstream_Space
http://halo.wikia.com/wiki/Slipspace_Anomaly