The spike drive is one of two methods of faster than light (FTL) travel that mankind has discovered, and the only one presently capable of being used.
The drive pushes a ship “upward” through a spectrum of dimensional frequencies, where it can travel faster than light. The ability to enter and exit these frequencies requires a massive gravity shadow, such as that produced by a star. As such, spike drive craft can only travel from one star system to another, travel to deep space using the spike drive is not possible.
Spike drive travel is potentially very dangerous due to varying currents in the metadimensions (the Scream was the equivalent of a tsunami in the metadimensions). If there is an established set of rutters for a particular route, then travel is relatively safe and easy, but charting those rutters in the first place can be extremely dangerous.
In addition to providing for FTL travel, the spike drive is also a factor in combat, used to phase a ship to a specific metadimension, which makes it difficult to damage except by a weapon attuned to fire into that specific metadimension.