Think of it as port forwarding...say your friend wanted to access your home PC from their place. You would need to add an entry in your home router to forward that traffic, and dst would be whatever internal IP your pc is, so say for rdp port would be 3389
Lets take same example here...lets pretend that somewhere from the Internet, someone has to reach your internal server on port 789
rule would be like this for nat:
original packet:src any, port 789, dst say your external IP
dst packet: src any, port 789, dst - your internal server
Makes sense?
And yes, R77.30 has been unsupported for ages now,please install at least R81, as even R80.40 will be unsupported next month 🙂