This is the Monument of Victor Emanuele II who is the first King of the the unified Italy. Italy all throughout history has been very segregated into Romans, Venetians, Florentians (and remains so today, just not as bad) so this is to commemorate the first king of a unifed Italy. It is called the the "wedding cake" by some who think it looks like something that goes on top of a wedding cake and others call it the "typewriter" apparently becuase of its profile view.