It means your level is below the recommended level for the item, and as such your only recieving part of the stat increases.
So, Dmg 16 (56) means that eventually the item will provide 56 damaage, but at the moment you are only getting 16 damage from it (due to your level being lower than recommended level)
atk is primarily based on str and dex. so the higher te better. Im not sure if one has a greater effect than the other or if dex maybe effects rangers more than palidans. I know that the effect of added strength seems to have a greater effect than added dex with my pali, but his str is 315 and dex is 307 so maybe at diminishing returns. BTW incas you wonder, worn atk us caped at 250 without aa