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.
You might be worried that you don't know when to use farther and when to use further, but that's because there's no hard and fast rule, just general habits. Farther is good for measurable things. If your blindfolded friend is missing the piñata, tell her, "go farther to the left." Further is good for abstractions, like when your teacher asks you to take an argument further. So: far for space, fur for ideas (and animals).
