Substance dualism and property dualism are two positions in the philosophy of mind, and they’re trying to answer questions like “Hey what’s the relationship between the mental and the physical?” or “what’s up with consciousness?" According to substance dualism, mental things and physical things are two totally different kinds of things. It would probably mean that no matter how much we study the brain, even if we managed to achieve a perfect understanding of the brain’s crazy intricate inner workings, it won’t be enough to understand the mind, or our mental experiences of subjective consciousness, because we’re looking at the wrong fucking thing. "Hey, quit studying that brain, it’s not even the right goddamn substance, you idiot.” Maybe if we figured out how the brain and the mind hook up, we’d learn something about the mind, but if substance dualism is true, then no amount of information about the brain would ever be enough to fully explain how the mind works.
According to property dualism, though, there’s only one kind of substance, and it just has different kinds of properties. So we only have to look at one thing, the brain, and that’s where we’d find both mental properties and physical properties. If we fully understood the brain, we’d have enough information to also explain but also, it would mean there’s a bunch more shit to understand about the brain than if substance dualism is true.
So, in short: substance dualism says “Oh no you’ve got the wrong thing entirely, stupid” and property dualism says “yeah, no, go on, keep looking at the brain, we’ll get it eventually."