Used about a phrase, a sentence, or a piece of writing that has been shaped with real care, so it sounds neat, elegant and satisfying. It is praise for the craft of the wording — the words are in a pleasing order, nothing is wasted, and you enjoy how it was said as much as what it said. You meet it almost only in the fixed pairing 'a well-turned phrase', and often in a slightly ironic way, when someone is good at sounding clever: 'He has a gift for the well-turned phrase, but he says very little.' The word feels literary and a bit old-fashioned; a student would read it far more often than say it. The picture behind it is a woodworker turning a piece of wood on a lathe until it is smooth and round — the writer 'turns' the sentence the same way. There is one other, much older use you may meet in old novels: 'a well-turned ankle', meaning a shapely leg. That one sounds dated now and can come across as odd, so leave it to the books.