Oneself is a reflexive pronoun that means “your own self” or “one's own self.” It's the formal way to talk about a person in general doing something to or for themselves.

You might read in a book that “one must apply oneself to difficult tasks” or “it's important to believe in oneself.” These sentences could also be written as “you must apply yourself” or “it's important to believe in yourself,” but oneself sounds more formal and refers to people in general rather than to you specifically.

The word comes up often in instructions or advice meant for anyone: “One should pace oneself during a marathon” means that any runner should manage their energy carefully. “To truly understand something, one must see it for oneself” means that people generally need firsthand experience, not just someone else's description.

You'll rarely hear oneself in casual conversation. Kids don't typically say “I taught oneself to skateboard”; they say “I taught myself.” But in formal writing, classic literature, or philosophical discussions, oneself appears frequently because it maintains a dignified distance while still talking about personal experience.