"The laws of physics are invariant in all inertial systems." is a statement. It's something somebody says to express an idea. Its not an object. An acquaintance who works in a restaurant once said "As long as nobody places any orders, we have everything completely under control." which basically says the exact same thing.
It means more than one thing.
It supports the notion that the laws of physics are only theory. As beautiful and absolute as they may sometimes seem, they do not define reality. They are merely a 'user level' interpretation of it. It being quite possible to successfully work with the laws of physics without understanding them. Nevertheless, their frame is by this statement limited to significantly inertial systems, since reality knows no such thing as a perfectly inertial system. The mentioned significance being something that can be calculated. The laws of physics are by this statement only invariably applicable on systems that can be considered absolutely inertial.
One could say the statement alerts to the fact that outside the realm of significantly inertial systems, the laws of physics will fail to give usable results.
It also means that as long as you have a grip on all (significant) dynamic factors in a system, you can count on the laws of physics to give you correct results that express a matching image of the experience of reality, even if you don't fully understand these laws.
It may also tell you, that if you find the laws of physics to apply successfully on a system, meaning any system at all, you're dealing with a (significantly) inertial system, even far outside the realm of physics.
Finally it tells you, that if you find the laws of physics to fail in giving you a matching image of reality and you want to bug-hunt that problem, what you need to look for is an unknown dynamic factor to add to your formulas and thereby to your image of reality.
Any law this does not apply on, is by this statement not to be interpreted as a "Law of Physics." The beauty of it is, that 'true' laws of physics can by the same precondition be successfully applied in fields other than physics.