But what’s contracting in relativity?

The "Ask a Physicist" blog explains:

Q: In relativity, length contracts at high speeds. But what’s contracting? Is it distance or space or is there even a difference? ...

This situation is sometimes explained as a consequence of length contraction. But what is it that’s contracting? Some authors put it down to space itself contracting, or just distance contracting (which it seems to me amounts to the same thing) and others say that’s nonsense because you could posit two spaceships heading in the same direction momentarily side by side and traveling at different speeds, so how can there be two different distances?

So what is the correct way to understand the situation from the astronaut’s perspective?

Physicist: Space and time don’t react to how you move around. They don’t contract or slow down just because you move fast relative to someone somewhere. What changes is how you perceive space and time. ...

Einstein’s big contribution (or one of them at least) was “combining” time and space under the umbrella of “spacetime”, so named because Germans love sticking words together

I agree with his explanation, except that the view he described was not Einstein's view.

Minkowski was the German who combined space and time into spacetime, and he based it on Poincare, not Einstein.

Einstein's contribution was not putting time and space together, and he very much disagreed with the view that what changes is our perception of space and time. I explain the point here and in my book, and in other posts. Einstein insisted that his view of the contraction was essentially the same as Lorentz's, and contrary to the non-Euclidean geometry view that is nicely explained in the above blog.