The U and V values are pretty much equivalent to X and Y positions, but for texture coordinates. They stand for the point on the texture that associates with a vertex. In Construct these coordinates are actually numbered like screen coordinates rather than values from 0.0 - 1.0.
Here's an image showing an example of UV coordinates:
Since Construct's sprite meshes require displacement values, these wouldn't be the actual values. The real values, starting with the left square, would be 0, 0 all the way around; the right square, on the other hand, would -- going from the top-left corner around clockwise -- have these values:
1 - 0, 0
2 - 128, 0
3 - 128, 128
4 - 0, 128
Hopefully I explained all of this well enough and that I got the main idea across. Hope this helps.