Sulfur, periodic table S, is a solid with 16 protons. It's most common isotope is S32 with 16 neutrons. Other stable isotopes exist in small amounts.
Sulfur is the tenth most common element by mass in the universe, and the fifth most common on Earth. Sulfur is common on the Martian surface. The Martian mantle is probably richer in sulfur, chlorine and phosphorus than Earth's.
Sulfur is one of the "CHNOPS" elements (Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, & Sulfur – the elements which are needed in quantity for life), so it will be required for life support. Fortunately, as settlers refine metals, they will generate significant amounts of sulfur as a waste product, so there are unlikely to be shortages of this element. (And if plants are grown are in local dirt, then will pick up sulfur naturally.)