From the Basics 1 notes
"Vala issa. Translated simply, it could mean "The man is", but that's not a very useful sentence. A better translation would be "He is a man", where "he" is simply not necessary."
From this I could assume that this sentence can mean both 'They are good knights' and 'The knights are good'. But I reckon if you stuck a pronoun in you could specify that it is indeed 'They are good knights' if you really needed to specify, but I'm guessing there's no difference and that they mean the same thing.
i haven't gotten very far in this yet, but i would assume that it has to do with word case. presumably "they are good swordsmen" uses the object case for swordsmen, but in "the swordsmen are good" you'd use the subject case for swordsmen. although i'm not sure what those cases look like in High Valyrian