I wrote this reply last night because I think the OP is asking a philosophical question, akin to What is an electron? I then deleted it, as a personal opinion on philosophical question.
I fully accept that the other descriptions are correct, because they have been tested to the limits of our measuring ability. But I still think an answer to this question based around coordinate systems and the differential geometry of a 4 D manifold, will eventually need to be modified if we find:
that space is discrete or
we are required to modify our current approach, to achieve a quantum theory of gravity.
In this way, we might eventually put the current philosophical question of what "is" space, into a mathematical form that acceptable to the physics community.
I think the other answers, apart from answer above (13526) in the comments, all unintentionally do not fully answer your question, because you are asking physicists to answer a philosophical question.
If I go downtown to have a coffee, taking 5 minutes to walk the mile to the cafe, that is a spacetime distance on a local scale, and it is the the way we think about distances (intervals in GR). And the same idea of space as a geometrical / coordinate system distance applies to as far we can observe looking outwards.
The problem arises when we try to view spacetime on a universal scale, when we don't have edges to the universe, (no matter whether K is equal to 1, greater than 1 or less than 1, that is that spacetime is either flat, closed or open).
We now have to find a different definition of spacetime because the local definition will not make any sense. The universe did not start at a point, it has no centre and it has no edges. Look up the number of times these questions have been asked here already.
So on a global scale, we (in my opinion), need to redefine spacetime as the relationship between objects in the universe and reconsider the idea that geometry and continuous 4D differentiable manifolds are the complete solution to the spacetime of the observable universe. Geometry may be of no use when describing an electron, and the beginning of the universe was "smaller" than that.
It is also not useful in describing the interior of black holes or if we discover that spacetime is actually discrete. (A long shot, I appreciate that).
So at both scales of the universe, at its "smallest", and at its "biggest", (remember we are in an expanding universe), the descriptions of the other answers of what space "is" fail us, they simply do not apply.
In summary, on a local scale, I believe that the geometry and coordinate system, while you may consider that they simply replace one word for another and avoid telling you what spacetime really "is", (because that is not the job of physics in the first place), they are essential if we are to keep track of our measurements on a local scale.
But, on a global / universal spacetime scale, I don't believe they deal fully in describing the observable universe from its "tiny" initial state, to its expanding edge state today.
A caveat here is that a closed universe may "kinda" have an edge, in that if you start at any point you may return to it, but the evidence points to a flat but expanding universe.
So I ask you to consider you a description of spacetime on a global scale as a set of relationships. I think this is more appropriate, in fact the only way, we can deal with what spacetime is, on a universal scale.
Using the handwavy word "relationship" brings in QM, which will need to be brought in anyway if we want to achieve a quantum gravity theory, but I have now moved from a personal opinion, to complete uninformed speculation. Time to stop.
My sincere apologies to those concerned, for the number of edits in this post, I will not repeat this practice in future.