It is spherical and it is the distance light could travel in the 13.7 billion years since the big bang, or more precisely since when the universe got cold enough to allow electrons and protons to form atoms which in turn made the universe transparent to light and which occurred about 400,000 years after the big bang. Note that the radius is about 46 billion light years and not only 13.7 billion light years because the universe continued to expand while the light was on its way to us. This also changed the frequency of the light and so the light emitted at the edge of the observable universe at the time of recombination is the cosmic microwave background we receive today.