We usually use farther to talk about a physical length or distance. Farther is the comparative form of far when talking about physical distance.
Examples:
1. If you walk farther down the road, you will see my house.
2. The school is farther down Oak Street.

We usually use further to talk about a metaphorical distance, like a distance in time. It refers to a time or amount.Further is another way of saying "additional" or "more time."
Examples:
1. His plan needs further study.
2. I won't listen to this any further.