The naming indeed partially explains itself.
So when we have for example a leaf of a tree model and UV map it and then copy the leaf multiple times, we going to get overlapping UV’s, meaning every leaf copy is going to get the same texture space on the UV map.
When we repack the UV’s so all the leafs get their own UV space, we get non-overlapping UV’s, meaning every leaf gets its own texture space on the map.
So yes, non-overlapping UV’s means every polygon has its own space on the map and with overlapping UV’s we have multiple polygons sharing the same texture space.
Note however its possible a model consists of two parts or more having multiple texture maps applied to it. 3D models can have multiple UV channels on them (for example, map 1 would be assigned to UV channel 1, map2 would be assigned to UV channel 2, etc.). In this case if you open such model in a UV map viewer you could possibly see the UV islands of those different channels superimposed on each other (getting impression of overlapping UV’s). In that case you would need to switch the different UV channels to view the singe UV layouts assigned on each channel.
