trueness of hadith about leaving what does not concern you; what is darussalam?

there are hadiths that say إِنَّ مِنْ حُسْنِ إِسْلاَمِ الْمَرْءِ تَرْكَهُ مَا لاَ يَعْنِيهِ that mean "Surely, of the excellence of a person's Islam is that he leaves what does not concern him.". this hadith is marked " Da'if (Darussalam) ". what is Darussalam? why/how they decided this hadith is weak (daif)? are not there similar hadithes which are not weak? this look like i like this hadith, but, actually, i think, how can this be true while quran and hadithes say that muslims should try to stop bad deeds? also, maybe this hadith easily can be used to say things like "do not learn that science, it does not concern you".