The holds a filament guide tube (e.g. bowden cable) on your Prusa.

You can purchase the guide here: http://www.mcmaster.com/#8547K23
I recommend a 2ft length.

• Slide it onto the protruding threaded rods on the top
• Slide in the filament guide tube
• Feed your filament through

(There is no need to mount the tube onto your extruder, it stays put pretty well by itself)