They are teak and if they've been left exposed for a couple of years, will be down to bare teak. You can leave them to weather grey or strip them down, sand lightly and refinish. If you search on teak finish on this site you'll find tons of discussion what is the best method to refinsh them. There are many different ways and materials to use. You'll have to remove them from the seat itself ( they're screwed from underneath) to do them properly.