This is the 377th most frequent Catalan word.
se
"Se" is a reflexive or impersonal pronoun in Catalan.
In this reflexive verb usage, 'es' is part of the verb 'rentar-se' (to wash oneself).
Es renta les mans abans de menjar.
He washes his hands before eating.
Here, 'se' is an impersonal pronoun used to express that the subject is not specific and refers generally.
Se sap que arribarà tard.
It is known that he will arrive late.
In this sentence, 'se' is used in a passive construction to indicate the action will be performed without indicating the actor.