A Kshatriya's pride, born of his nature, lives in valor, exuberance, determination, resourcefulness, bravery in battle, and generosity and noble demeanor as a lord of men. A Vaishya's task, born with his nature, is to till the ground, tend cattle, venture trade. A Shudra's state, suiting his nature, is service, arguably in service, to the other three classes. Now once again, it doesn't make strict reference to you are born a Brahmin. If your father was a Brahmin, then you have to be a Brahmin, or if your father was a Kshatriya, you will be a Kshatriya. It is making reference to this idea of being born of his nature. So you could interpret that as this idea that you have some innate gifts, you have some innate tendencies, and those innate tendencies are going to dictate whether you're going to be a Brahmin, a Kshatriya, a Vaishya, or a Shudra.