Euclid aims to accurately map galaxy redshift. Redshift is one of the primary ways we have to turn a picture of the night sky into something more like a 3D map that has depth. It's based around something very much like the Doppler shift. And the final part of this jigsaw, I suppose, is that the universe is expanding. What that means is that as you look at things that are further and further away, they are receding from us faster and faster.