Fluorocarbon is a type of nylon line that appears more invisible underwater than other lines, and it sinks. Fluorocarbons can be hard to cast with in small spinning reel outfits, but are very effective for finicky fish. Braided lines are effective because they have a better strength/line diameter comparison. You can fish a smaller diameter line and still have the same the same strength as a mono of a bigger size. This can lead to burying the line in the spool when setting the hook, and eventually backlash. But this can be avoided by setting the drag lower. Also braided line doesn’t stretch near as much as fluorocarbon line. Both lines are more abrasion resistant than monofilament, and typically last longer.