"Indeed every monad must be different from every other. For there are never in nature two beings, which are precisely alike, and in which it is not possible to find some difference which is internal, or based on some intrinsic quality"

The quote indicates that every entity in nature, whether it be a living being or a things, is unique and various from every other. Even if 2 beings seem similar on the surface area, there is constantly some intrinsic quality or internal difference that separates them. To put it simply, every existing thing is unique and has its own distinct attributes that set it apart from any other thing. This is the principle of individuality that Leibniz thought in, mentioning that every entity or "monad" has its own unique identity.