Back-focusing/front-focusing is when your auto-focus is wrong, and focuses too far or too close. This is most visible when you're shotting with fast telephoto lenses and in other situations with shallow depth of field. Most people don't like to have their photo out-of-focus, so if you're suffering from this it's best to fix it.
As Matt writes, you can check your camera+lens using chart at focustestchart.com. On some cameras front-back focusing can be tuned directly in their menu, but sometimes it's necessary to get the camera and/or lens calibrated in service center.