To expand on this further, if the universe is closed (has positive curvature) and the shape of space is spherical, this would mean that initially parallel beams of light would converge (like how, when you follow two lines which are parallel at the equator on a globe, they meet up at the north pole). If the universe is flat (has zero curvature) and the shape of space is flat, then parallel beams of light would remain parallel. Finally, if the universe is open (has negative curvature) and the shape of space is hyperbolic (like a saddle) then initially parallel beams of light would diverge (spread apart).