Federalist 78 what does hamilton mean when he says that an "independent spirit in the judges" is essential for them to do their duty

Basically it means that the rulings of judges should be free from outside influence. Federal judges are given life tenure unless they mess up REALLY badly. If judges were subjected to political retaliation for an unpopular decision, then they may be subjected to outside pressures. However, because federal judges sit on the bench until they die or choose to retire, they do not have to worry about any backlash from an unpopular decision and they are free to concentrate on making a correct ruling rather than worrying about whether they will lose their job.

the spanish  

hope this meaning spain btw