It’s a way to refer to and separate permissions.
You can create any group you like.
The system however, is set up to create certain files
with certain permissions (user, group, read, write, execute …)
(/dev/ttyUSB0 in your case)
To access those files, to read and write, is only possible when the permissions allow that.
That device file will be accessible (by default) by members of the group “uucp” on Arch like systems and “dialout” on Debian like systems.
Your user account being a part of the group allows you to access it - otherwise only root could do that.
It could help just having a look at the file and it’s permissions.
ls -al /dev/ttyUSB*
(it only gets created once the device is actually connected)
Issue solved – It turns out that at least part of the problem may have been a hardware glitch with the GPS.
One thing that worked was explicitly typing in my connection into the serial port dropdown list. It is supposed to autofill with available connections but I found out that it was possible to manually enter /dev/rfcomm0. This got OpenCPN to at least see some serial data except the sentences were messed up.
I took the GPS outside to see if it would settle. It would not UNTIL I unplugged the power cable when I did this the GPS settled and the nmea sentences started making sense. It still worked OK after I plugged the cable back in.
Thank you for the assistance and it was a hell of a learning experience. Way harder than it should have been but I would still be struggling without your help. All the best.