Natures increase one stat by 10% and decrease one stat by 10%. "Neutral" natures are ones that increase and decrease the same stat, so the effect is nullified.
See http://pokemondb.net/nature for a table of natures and which stats they increase/decrease.
When you're working out the highest possible value a stat can be you need a nature that increases said stat. So for the attack stat, "beneficial" means the pokemon would have a nature that increases attack, like Lonely or Adamant.
"Hindering" is the opposite - to get the lowest possible value you'd need a nature that reduces that stat.
Remember, the actual maximum is incredibly difficult to get near to, since you need max IVs, good nature and max EVs. Obviously it's impossible to get the max value across all stats.
