Nail polish top coat goes on like regular nail polish but is completely transparent. It dries shiny and super hard. This is an extra step that will make your nail polish stronger and less prone to chipping. It will also keep it shinier longer. If you do plan to use a top coat, consider choosing a complementary product. Most nail polish manufacturers produce their own lines of top coat -- which are optimized for their nail polish. When you stock up on nail supplies, keep that in mind.
Nail polish top coat has another nice feature: You can reapply it as needed. If you've ever noticed your polish dulling down between treatments, a shiny refresher may be just the thing to keep your nails glossy until you're ready to change your color from blue to fire engine red.