A fighting butt is a small extension (approx 2 to 3 inches) to the rod handle, sometimes removable.
They help as they can provide extra support for the rod while the handle extension lies along your arm, or, if you have the "biggest fish you've ever caught in your entire life" on the end of the line, you can more comfortably rest the rod butt in your stomach.
Personally I find they take a bit of getting used to as they I had to alter my casting style slightly to accommodate it.
Hope that helps.