What does a light year measure?

Distance, not time - how far light travels in a year, about 9.5 trillion kilometres. The word year inside it does the damage, and it is one of the most reliably misunderstood units in science.

It exists because the alternatives are unusable. The nearest star after the Sun is roughly 40 trillion kilometres away, and the numbers stop meaning anything very quickly; four light years is a quantity a person can hold. Professional astronomers actually tend to use the parsec, which is about 3.26 light years and comes out of the geometry of measuring stellar distances by parallax.

The consequence is that distance and time become the same measurement. Looking four light years out is looking four years back. The Andromeda galaxy is two and a half million light years away, so it is being seen as it was before our species existed, and the most distant galaxies photographed by the James Webb telescope are being seen as they were within a few hundred million years of the Big Bang. There is no way to see anything in space as it is now.