How do Cargo features work, and when should beginners start using them?

Learn to love:

cargo add serde -F derive

That's usually the first encounter with feature flags (often alongside "Why didn't deriving Serialize work?").

Feature flags are defined in Cargo.toml, and can affect both the dependencies that are pulled in by a crate, and what functions and types are included. derive is a common one, because proc-macros wind up in a separate crate.

For beginners, this can be daunting: what features do I need? You'll probably wind up typing cargo add tokio -F full because figuring them out is tricky!

So my advice is that you shouldn't add feature flags to your crate until you really need them. The first time I used them, I needed to support multiple back-ends for a rendering system (OpenGL, wgpu, etc.).