The word si has two meaning: the main meaning is as a 3rd person reflexive pronoun Pietro si lava le mani. Peter washes his hands. the 2nd meaning, by extension, is as an indefinite subject “someone” or “people” or “one.” Si mangia molto. One eats a lot / a lot is eaten / etc. Occasionally, we want to use both meanings. In order to avoid repeating the “si,” we say “ci si” One washes one’s hands. Ci si lava le mani. Note: The literal meaning of “ci” was “here” in Latin, and in Italian “we” or “here.”

Actually 'Ci' is a personal pronoun, used also to make an impersonal sentence and 'Si' stands for 'lavarSI' (reflexive verb) Te translation could be 'people wash their hands'