Why does this Ring have 1930 one florin fid def inscribed inside?

It was common for amateur jewelers to make rings by hammering (or otherwise manipulating) silver coins. The inscription indicates that it was a British 2-shilling piece, normally called by the unusual name of "florin".

The practice of making hammered coin rings has pretty much stopped because circulation coins are no longer silver, and they're worth much more for their metal or collector's value than as rings.