A Knock Box, Knock Tube, Knockout Box or Knockout Tube -different names for the same item - is a device used to store spent espresso grounds after a shot of espresso has been pulled. It is generally made out of sturdy material and has a strong bar (bash bar) against which a portafilter is tapped to release the puck. Because the espresso puck is typically very hot after extraction, knock boxes need to be sturdy. Furthermore, they are also subject to repetitive knocking and tapping to dislodge espresso after use, and thus rubber, plastics like polypropylene and stainless steel are used. Knock boxes are usually located near the grinder for convenience.
Used coffee grounds are not just waste but a "green" nitrogen (C/N ratio about 20:1), magnesium and calcium source and so an excellent addition to the garden compost. You can also simply spread it around your plants. Earthworms are very attracted to used coffee grounds, slugs however are repelled.