'Farther' means at a greater distance - it's the comparative form of 'far,' used for physical distance. For example, 'my house is farther from school than yours' means there is more distance between my house and school. Many English speakers also just use 'further' for distance too, so you'll see both - but if you want to be careful, 'farther' is for physical distance, and 'further' is more for other kinds of distance, like ideas or amounts.