The "ball" is the glide path indicator on the left side of the angle deck of a carrier.  It indicates above/below glide path and wing attitude.  Pilots coming in for landing "call the ball" when they have visual of the indicators. The ball is used on carriers because there is no instrumentation that can deal with the rock & roll of even the largest of carriers if seas are heavy, and there aren't pilots that can make necessary adjustments visually.  Night time, it's virtually impossible to land on a carrier without the indicators.