When you sign in as an administrator you may see this prompt and trigger an email to your registered address. This has replaced the need to specify and maintain IP ranges on administrator accounts.
Copy the code from the email into the box and you'll be signed in.
This will cover you for up to 5 days at your current location, but you may see it more often if your location appears to change - e.g. you change browser or clear cookies. If your browser of choice has an option to clear cookies when closed, you may want to set that to off.
Codes can only be used once and must be used within an hour (from the time they're generated). If you're confident that neither of these are the reason a code hasn't worked for you, contact your usual support provider.