Sulfur and Fluorine are nonmetals so they will form covalent bonds to gain stability. To do so, they will follow the octet rule: they will share enough electrons so as to have their valence shell complete with 8 electrons.
Sulfur is in the Group 16 in the Periodic Table and has 6 valence electrons. Thus it must share 2 pairs of electrons to reach the octet.
Fluorine is in the Group 17 in the Periodic Table so each F has 7 valence electrons. Thus, each F needs to share 1 pair of electrons to reach the octet.
As a consequence, they will be bonded in the order F - S - F, with a single bond between each pair of atoms.