Many people would take this Maxim to mean just what is yours in the physical world, however it is much more than that. What is yours also includes things like your integrity, your sense of self-worth, and your knowledge. It is these things that are the most important to guard so that you may, in time, pass them on to future generations – if in no other way, by your example.
So, what does it mean to guard these things? It means that you must not allow people to degrade and disparage their worth. It means that you must take care to not degrade others as well because your integrity will suffer. It means that you will not allow yourself to believe you are any less because either someone is better than you at a thing or someone tells you that you are not worthy. You must not take insults to heart as it only proves that the person doing the insulting is afraid of your self-worth and wishes to diminish it. In fact, you must daily do things that help you feel good about yourself so that you may keep your self-worth intact.
Most importantly, you must safeguard your knowledge. Do not be afraid to have a good library for reference or to actually write things down in a physical form. Do not worry if your writing is not good enough for anyone else to read, just do it for your own sake and for the sake of what you are committing to the page.
When you do these things, in my opinion, you are living the heart of this Maxim. Add that to safeguarding your material possessions and work to live as a free person should.