A puck screen (contact or shower screen) is a thin steel mesh disc that sits on top of the tamped puck. It spreads the water arriving from the group more evenly, reduces channeling, and keeps the machine's own shower screen cleaner. It is sized to drop inside the basket, so a screen is chosen by the basket's internal diameter — a 58mm machine takes a ~58.5mm screen, and it is a near-universal accessory within a size.
New to sizing? See espresso portafilter sizes explained, or browse all part types.