Hi, The 's' means the verb is directed to yourself. So for instance "se cacher" would mean to hide yourself whereas "cacher" means to hide something or someone else. Here "arrêter de fumer" and "s'arrêter de fumer" both refer to yourself and I can't think of any difference in meaning. However in imperative sentences "s'arrêter" doesn't sound as natural as "arrêter" to me. So when telling your dad "Quit smoking !" you better say "Arrête de fumer!" Hope this helps

In French, many verbs have 2 forms : the usual form (ici, "arrêter") and the pronominal form ("s'arrêter"). The more usual form can be combined with a "complément d'objet direct" : "la police arrête le voleur". The "pronominal" form must go with the "pronom personnel" and nothing else. Dans la phrase suivante : "je roulais trop vite, je n'ai pas pu m'arrêter", "m'", which stands for "me".