Farther means a greater distance away. You thought you'd be able to hit that can with a rock, but it fell short, so you'll have to try throwing it farther.
The word farther is good for measurable things, things that are far far away or at least farther than something else. If your blindfolded friend is missing the piñata, tell her, "go farther to the left." It’s often confused with further, which is good for abstractions, like when your teacher asks you to take an argument further. So use far for space, and fur for ideas (and animals).