What is deck cleanout on a lawn mower?

The deck on a lawn mower is the shaped metal shroud that keeps the spinning blade(s) from being exposed to the user during operation. Many manufactures use the shape of the deck to create a mild vacuum that draws the blades of grass up toward the mower blade(s). After mowing the lawn, some grass will be stuck on the underside of the deck. If the lawn is somewhat long and wet, the grass will build up on the inner surface of the deck to a point where only the blade(s) have clearance. This stops the deck shape from performing its designed function.

Deck cleanout is performed while the mower is NOT running. Turn the mower on its side and scrape the excess grass from out of the underside of the deck.