When introducing the SWD interface, ARM also introduced an optional extension for SWD, called Serial Wire Output (SWO). SWO specifies a dedicated pin, in addition to the debug signals of the SWD interface, which allows the target CPU to output specific data like printf output on a dedicates pin via UART or Manchester protocol. This pin is unidirectional, so it is not possible to send data to the target CPU on this pin.
Note: Not all targets that support SWD also support SWO. Moreover, SWO is not supported by all CPU architectures that support SWD.