Entry into hyperspace is achieved when a vessel opens a stable hyperspace window. This allows a vessel to enter subspace and achieve faster than light velocities relative to real space whilst traveling at sublight speeds within its own layer of subspace. In this state, they can fly from one point to another in a relatively straight line. Hyperspace gives off high levels of radiation, but this is only a problem for the hulls of Wraith vessels, which are organic and unshielded.
More: Hyperspace by SG Command