I looked this up--  The Landing Signal Officer may ask the pilot to call the ball, which means asking the pilot to verify that he sees the navigational signal light on board the carrier indicating whether the pilot is on correct approach path to landing.  The 'ball' will appear in different colors to indicate too low, too high or just right.  The pilot responds including some other information, including the amount of fuel remaining, in case the landing cannot be made. There are also some green navigation lights to the right and left of the 'ball' , which the LSO may flash to signify "Roger Ball."