At the moment, don't worry about it too much, mate. Try reading through the relevant sections in the Prelim Leading Edge Textbook (Riley's explanation of it may overcomplicate it a tad). If it's still hazy, wait until next year, because when you come to the section External Stability, in Topic 3 – Economic Issues, you will learn about NFL (net foreign liabilities), NFD (net foreign debt) and NFE (net foreign equity) again, as they are important concepts regarding external stability. Anyway, when your teacher covers it again, try understanding it as much as possible and ask as many questions as necessary to iron out any confusion that you have. After that, read through Dixon’s (Leading Edge) explanation in the textbook you will be using for the HSC course, and then to consolidate, try reading through the explanation in the Riley textbook for the HSC course. If you’re still having problems, then try asking your teacher again or do some research online or, of course, post on BOS asking for clarification on any confusion you have.
Good luck, in the meantime