Hi there,
And Welcome Here…
SO The Nrf52840 Has 2 Serial UARTS on the silicon, Serial & Serial1 and There is also the possibility to use a GPIO pin to “Bit_Bang” the data into the MCU pins (emulating a serial port functionality) Software Serial is that LIBRARY that enables that to work on a set of GPIO pins.
There is a Speed limitation on this method.
Search software serial and you will find all the info you should ever need, also some of the Xiao wiki’s use this tech to connect to certain peripherals. 
HTH
GL 
PJ 