What does Keep me signed in do?
It keeps you signed in after you close the browser or the app.
When it is on
You are still signed in when you come back. This lasts until the saved sign-in runs out or you sign out.
Each time SOS opens, it checks your saved sign-in and renews it.
When it is off
On the website, your sign-in lasts for that browser session. Closing the browser ends it, and reloading the page can too.
In a phone app, your sign-in ends when you close the app fully and open it again.
On a shared device
Untick the box, and tap Sign Out in Settings when you finish.
Signing out also removes a saved Face ID or fingerprint sign-in from that device.