Oneself is the reflexive form of one — the same job as myself after I, or yourself after you. You use it when the person you mean is that careful, general one: "One should be kind to oneself." It can also put weight on the idea that you do the thing, not someone else: to do it oneself.

It is quite formal, and it belongs with the pronoun one. Textbooks like it. Though actually, everyday English often runs away from one altogether. Many people say you instead: "You have to be kind to yourself." If your subject is I, she, they, or we, oneself is the wrong word; you need myself, herself, themselves, or ourselves.

Older writing sometimes has one's self as two words. Today we write oneself as one word.