It’s not entirely clear, and the explanation varies based on the source you look at.
There are only a few things that are generally consistent about hyperspace across all major canon:
It allows FTL travel in some way.
For most races, it requires special technology to utilize it, which is lacking on some starships. Two notable examples of ships lacking a ...
It is a 'dimension of space-time', 'coterminous with realspace'.
For those who are unfamiliar with hyperspace, it is
a dimension of space-time that can be entered only at
faster-than-light speeds. While the most knowledgeable
astrophysicists and astrogation experts admit that
some aspects of hyperspace remain a mystery, it is
understood that hyperspace is ...
From the original trilogy, it is evident that "hyperspace" is just a name, and the travel does not appear to take place outside of normal space. There are are at least two definite indications of this.
First, note that as ships accelerate to hyperspace speeds, they see the stars stretch out, indicating that they have reached faster-than-light ...