In the Galaxy S line, it was just a way of indicating that it was newer than the original Samsung Galaxy
. I don't think it particuarly meant anything, it was just a marketing gimmick.
Then the Galaxy S got popular, so every other manufacturer half-inched the name in hopes of leeching sales for their low-to-medium end smartphones
, presumably on the grounds that it would be difficult for Samsung to defend a trademark on the letter "S". Even Samsung itself did it with its cunningly named, but distinctly entry-level soundalike smartphone the "Galaxy Ace".