Why PTT stands for Push-To-Talk. The PTT line is a control input that keys the radio: pulling it to ground (usually through a microphone switch, foot switch, or an external device such as an amplifier or computer interface) tells the transceiver to switch from receive to transmit. It is an active-low control line, so opening the circuit returns the radio to receive.
Watch out A CW key connects to the separate KEY jack; that also causes transmission, but the PTT line is the general transmit-enable control used for voice and data modes, not for forming code characters.
PTT = Push To Talk. Ground it and you're transmitting; release it and you're listening.