The theory was first proposed in 1995 by Robert Bauval, and has since been expanded in collaboration with Adrian Gilbert and Graham Hancock to include the Great Sphinx as a representation of the constellation Leo, and the Nile river to represent the Milky Way galaxy. This theory not only notes that there is a match between these arrangements, but postulates that this was deliberate on the part of the builders of the pyramids. That sounds OK, but the biggie here is that the stars in Orion's Belt and the constellation of Leo match up in these locations approximately 10,500 B.C.E., a mere 8,000 years earlier than the accepted accounts!
Well, despite some support from geologists who have argued that the effects of water erosion on the Great Sphinx date parts of that monument to at least 7,000-5,000 B.C.E., mainstream Egyptologists don't accept the Orion thesis. It does put the Great Pyramids and the Great Sphinx into an era before the Ancient Egyptians, so that's understandable! Still, it's a fascinating thought that someone might have built these huge monuments in relation to the constellations.
And, here I have to note that my daughter was quite thrilled by this idea when studying ancient cultures, probably in elementary school. She did a project on this very hypothesis, building a 3-D map of the Pyramids with the Great Sphinx, the Nile and all carefully positioned to match up with a picture of Orion. It was remarkably cool!