When a user logs in and has checked the "Remember Me" checkbox, a cookie is set in that that browser, on that computer (or phone or other device).  The user will automatically be logged in when returning to the site, using the same browser and device for the next 30 days.  After 30 days, that cookie will expire and the user will be asked to log in again.  This is a security precaution that is a common practice on the web.  If the user uses any other browser or device, they will have to enter their login information.  It is highly recommended that a user keeps track of their login information and does not solely rely on the "Remember Me" checkbox indefinitely.

If a user clears their browsing history, this will delete the cookie from the device which would result in the user needing to enter their login information upon the next login.  Also, browser add-ons have the potential of deleting cookies.

Using incognito mode starts a new browser session that has no cookies, therefore rendering the "Remember Me" cookie moot.  If a browser is auto-filling a password in incognito mode, that is a feature of the browser, not of the News!Site Software.  Many browsers prompt the user with the option to remember their username and password.  This is a browser features that is completely separate from the News!Site Software.