Saved login and cookie state. Create a profile once, reference it by id forever.
A browser profile is saved login/cookie state — the “stay logged in” of a real browser. It’s a standalone, project-owned resource: you create it once, then reference it by id whenever you want a browser or an agent run to start already logged in.
A new profile is empty. It gets its state the first time it’s used: the agent (or you, over CDP) logs in once inside a browser started with this profile, and when that run or browser ends, the session is saved back into the profile for next time. See Use a profile for how to attach it and how persistence works.