What do shape of universe mean? What shape does a three dimensional universe have ? and how did scientists determined that it is flat and not curved.
I think it's useful to realize the meaning of "shape" is not the same as in "what is the shape of this object I'm holding." Because we are a part of the universe, "shape" is about how things work (eg, how light moves through the universe), rather than "is the universe a sphere, cube, hypersphere or some other 'shape'."

This question is probably too broad to be answered here. I suggest reading the WikiPedia article on the 'Shape of the Universe'.