When on approach and “in the groove” (i.e. 15-18 seconds to touchdown) to an aircraft carrier the Landing Signal Officer (LSO) onboard will say to the approaching aircraft, “Call the ball.”
What he is asking for are a couple of things, but primarily if the pilot can see the round orange "meatball" on the Optical Landing System (OLS) as Tim Morgan has well described. If he cannot see the ball for whatever reason, the pilot will transmit, “Clara.” This tells the LSO that the pilot is not receiving optical glide slope information, a situation that must be corrected by the pilot or LSO quickly. Otherwise a wave-off is required.
Most all of the time though, the pilot will see the ball (and of course the related lighting system) and will note the orange ball's relation to the green horizontal datum lights. This will indicate if he is high, low, or on glide-slope.
In response to the LSO’s query to “Call the ball,” the pilot will answer by stating his side number, aircraft type, ‘ball’, and fuel state (and qualification number if in training).
The landing has been seen and called back to the top with a green light to bring her in.
HERE WE GO BOYS