Use the word farther when you mean physical distance, like if you run farther than your friend. Use further for basically everything else. Further refers to abstractions like ideas or thoughts.
Farther is more restrictive because it refers to physical distance. It’s got the word "far" in there to help you remember. Here are some examples:
"In fact, on every measure, those who run farther, faster or more frequently recorded a lower death rate than non-runners." (New York Times)
"The pilots traveled farther and longer in a gas balloon than anyone in history." (Washington Times)
Furthermore, further refers to figurative distance. It’s not the kind you measure with a ruler but like when you think further about something. It’s the older word of the two, and often does double duty. Here are some examples:
"But the agency sought to go further, by exploring the reasons for those deficiencies." (New York Times)
"The app takes the Tinder model one step further by actually optimizing possible matches according to consumption preferences." (Time)
Some people think farther and further are the same, but nothing could be further from the truth! (See what we did there?) Historically, they were interchangeable, but like friends growing further apart, they now each have their own meaning. Farther is used literally, and further is used figuratively.