Farthest means at the greatest distance. The farthest point, farthest from home, travelled farthest.
It is the superlative of far, and furthest does the same job — the rule taught is that farthest is for literal distance and furthest for everything else, but both are used for both and neither is wrong. Note further, farther; and that far is one of the few English adjectives with two competing comparative forms.
1adjectivefarthestAt the greatest distance.
“They pitched their tent in the farthest corner of the field, away from the road.”
farthest is a comparison of distance.