It means that the ring is made of 10 karat gold and "FL" is the mark of the manufacturer.
10K indicates that the item is 41.7% gold with the remaining being other metals (such as silver or nickel). While this lowers the cost of the piece, it actually makes the jewelry stronger.
Check out makers who use F. The identification would then depend on the style of the letter and any other marks around it.
And unlike trademarks that have been registered with the U.S. Patent Office or with another country's patent office, maker's mark are unregistered. If you are unfamiliar with the name or symbol, consult a guide to identify the maker.
Learn 8 Things You Should Know Before Buying Gold or Silver Jewelry, featured on Yahoo.com.