What is the function of a transceiver’s PTT input?

PTT stands for Push-To-Talk. The PTT circuit is essentially a switch that commands the radio to change its state from listening (receive) to sending (transmit). In almost all modern amateur radio equipment, this is achieved by grounding the PTT line to complete the control circuit. When you press the button on the side of a microphone, you are mechanically closing this circuit to ground. The other options describe completely different functions. A CW key input is specifically for Morse code, and tuning tones are used for antenna tuning, neither of which represents the primary function of a PTT input.